diff --git a/Make.inc b/Make.inc
index 2cd5797e88a41..fe7236a328d12 100644
--- a/Make.inc
+++ b/Make.inc
@@ -80,9 +80,6 @@ HAVE_SSP := 0
 WITH_GC_VERIFY := 0
 WITH_GC_DEBUG_ENV := 0
 
-# Use stock if MMTK_PLAN hasn't been defined
-MMTK_PLAN ?= None
-
 # Enable DTrace support
 WITH_DTRACE := 0
 
@@ -836,11 +833,20 @@ JCXXFLAGS += -DGC_DEBUG_ENV
 JCFLAGS += -DGC_DEBUG_ENV
 endif
 
-ifneq (${MMTK_PLAN},None)
-JCXXFLAGS += -DMMTK_GC
-JCFLAGS += -DMMTK_GC
-# Do a release build on the binding by default
-MMTK_BUILD ?= release
+# By default we use the stock GC
+WITH_THIRD_PARTY_GC ?= 0
+
+# Convert to lowercase
+USE_THIRD_PARTY_GC := $(shell echo $(WITH_THIRD_PARTY_GC) | tr A-Z a-z)
+
+# NB: When adding a new GC, make sure to add -DWITH_THIRD_PARTY_HEAP=<#NEW_GC>
+# to the variables JCFLAGS and JCXXFLAGS according to the mapping (MMTK=1, #NEW_GC=2)
+
+ifeq (${USE_THIRD_PARTY_GC},mmtk)
+JCXXFLAGS += -DWITH_THIRD_PARTY_HEAP=1
+JCFLAGS += -DWITH_THIRD_PARTY_HEAP=1
+
+# Must specify a supported MMTk Plan: Immix or StickyImmix
 ifeq (${MMTK_PLAN},Immix)
 JCXXFLAGS += -DMMTK_PLAN_IMMIX
 JCFLAGS += -DMMTK_PLAN_IMMIX
@@ -848,22 +854,21 @@ else ifeq (${MMTK_PLAN},StickyImmix)
 JCXXFLAGS += -DMMTK_PLAN_STICKYIMMIX
 JCFLAGS += -DMMTK_PLAN_STICKYIMMIX
 else
-$(error "Unsupported MMTk plan: $(MMTK_PLAN)")
+$(error "Unsupported MMTk plan: $(MMTK_PLAN). Supported plans: Immix or StickyImmix.")
 endif
 
+# Do a release build on the binding by default
+MMTK_BUILD ?= release
+
 # Location of mmtk-julia binding
 # (needed for api/*.h and .so file)
 MMTK_JULIA_DIR ?= $(BUILDROOT)/usr/lib/mmtk_julia
-
 MMTK_DIR = ${MMTK_JULIA_DIR}/mmtk
 MMTK_API_INC = ${MMTK_DIR}/api
-
 MMTK_LIB := -lmmtk_julia
-else
-MMTK_JULIA_INC :=
-MMTK_LIB :=
 endif
 
+
 ifeq ($(WITH_DTRACE), 1)
 JCXXFLAGS += -DUSE_DTRACE
 JCFLAGS += -DUSE_DTRACE
@@ -964,19 +969,17 @@ ARCH := $(BUILD_OS)
 endif
 endif
 
-# MMTk is only available on x86_64 Linux for now
-ifneq (${MMTK_PLAN},None)
 
+ifeq (${USE_THIRD_PARTY_GC},mmtk)
+# MMTk is only available on x86_64 Linux for now
 ifeq ($(OS),Linux)
 MMTK_LIB_NAME := libmmtk_julia.so
 else
 $(error "Unsupported OS for MMTk")
 endif
-
 ifneq ($(ARCH),x86_64)
 $(error "Unsupported build architecture for MMTk")
 endif
-
 endif
 
 # Detect common pre-SSE2 JULIA_CPU_TARGET values known not to work (#7185)
@@ -1405,7 +1408,12 @@ CSL_NEXT_GLIBCXX_VERSION=GLIBCXX_3\.4\.34|GLIBCXX_3\.5\.|GLIBCXX_4\.
 # Note: we explicitly _do not_ define `CSL` here, since it requires some more
 # advanced techniques to decide whether it should be installed from a BB source
 # or not.  See `deps/csl.mk` for more detail.
-BB_PROJECTS := BLASTRAMPOLINE OPENBLAS LLVM LIBSUITESPARSE OPENLIBM GMP OPENSSL LIBSSH2 NGHTTP2 MPFR CURL LIBGIT2 PCRE LIBUV LIBUNWIND DSFMT OBJCONV ZLIB P7ZIP LLD LIBTRACYCLIENT BOLT MMTK_JULIA
+BB_PROJECTS := BLASTRAMPOLINE OPENBLAS LLVM LIBSUITESPARSE OPENLIBM GMP OPENSSL LIBSSH2 NGHTTP2 MPFR CURL LIBGIT2 PCRE LIBUV LIBUNWIND DSFMT OBJCONV ZLIB P7ZIP LLD LIBTRACYCLIENT BOLT
+
+ifeq (${USE_THIRD_PARTY_GC},mmtk)
+BB_PROJECTS += MMTK_JULIA
+endif
+
 define SET_BB_DEFAULT
 # First, check to see if BB is disabled on a global setting
 ifeq ($$(USE_BINARYBUILDER),0)
diff --git a/Makefile b/Makefile
index 0f1e8c45edf40..a9f23592ec631 100644
--- a/Makefile
+++ b/Makefile
@@ -281,7 +281,7 @@ endif
 endif
 endif
 
-ifneq (${MMTK_PLAN},None)
+ifeq (${USE_THIRD_PARTY_GC},mmtk)
 # Make sure we use the right version of $MMTK_PLAN, $MMTK_MOVING and $MMTK_BUILD
 # if we use the BinaryBuilder version of mmtk-julia
 ifeq ($(USE_BINARYBUILDER_MMTK_JULIA),1)
@@ -291,10 +291,10 @@ else ifeq (${MMTK_PLAN},StickyImmix)
 LIB_PATH_PLAN = sticky
 endif
 
-ifeq ($(MMTK_MOVING), 0)
-LIB_PATH_MOVING := non_moving
-else
+ifeq ($(MMTK_MOVING), 1)
 LIB_PATH_MOVING := moving
+else
+LIB_PATH_MOVING := non_moving
 endif
 
 JL_PRIVATE_LIBS-0 += $(LIB_PATH_PLAN)/$(LIB_PATH_MOVING)/$(MMTK_BUILD)/libmmtk_julia
diff --git a/deps/Makefile b/deps/Makefile
index 396b1021c2ddd..4ad4df9ec4ba4 100644
--- a/deps/Makefile
+++ b/deps/Makefile
@@ -195,7 +195,7 @@ DEP_LIBS += libwhich
 endif
 endif
 
-ifneq (${MMTK_PLAN},None)
+ifeq (${USE_THIRD_PARTY_GC},mmtk)
 DEP_LIBS += mmtk_julia
 endif
 
diff --git a/doc/src/devdocs/gc-mmtk.md b/doc/src/devdocs/gc-mmtk.md
index 323e7901b6d2f..cd877e074ec34 100644
--- a/doc/src/devdocs/gc-mmtk.md
+++ b/doc/src/devdocs/gc-mmtk.md
@@ -7,7 +7,7 @@ There has been quite a lot of effort to refactor the GC code inside Julia to sup
 
 ## Building Julia with MMTk
 
-There are 3 different ways of building Julia with MMTk: building from source using a fixed release of the binding, checking out a custom version in the mmtk-julia [repository](https://github.com/mmtk/mmtk-julia) or using a precompiled binary from Julia's BinaryBuilder. The easiest way is to use the BinaryBuilder binary. Simply set the variable `MMTK_PLAN` to one of the supported plans below and build Julia as usual.
+There are 3 different ways of building Julia with MMTk: building from source using a fixed release of the binding, checking out a custom version in the mmtk-julia [repository](https://github.com/mmtk/mmtk-julia) or using a precompiled binary from Julia's BinaryBuilder. The easiest way is to use the BinaryBuilder binary. First, to enable MMTk as a third-party GC, set the variable `WITH_THIRD_PARTY_GC` to `mmtk`. Then, simply set the variable `MMTK_PLAN` to one of the supported plans below and build Julia as usual.
 
 There are different configurations supported by the following variables, which can be set in a `Make.user` file or as an environment variable.
 
diff --git a/src/Makefile b/src/Makefile
index 130c35960f2d0..a4a6d6f54d7fb 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -29,7 +29,7 @@ ifeq ($(USECLANG),1)
 FLAGS += -Wno-return-type-c-linkage -Wno-atomic-alignment
 endif
 
-ifneq (${MMTK_PLAN},None)
+ifeq (${USE_THIRD_PARTY_GC},mmtk)
 FLAGS += -I$(MMTK_API_INC)
 endif
 
@@ -46,7 +46,7 @@ endif
 
 # GC source code. It depends on which GC implementation to use.
 GC_SRCS := gc-common gc-stacks gc-alloc-profiler gc-heap-snapshot
-ifneq (${MMTK_PLAN},None)
+ifeq (${USE_THIRD_PARTY_GC},mmtk)
 GC_SRCS += gc-mmtk
 else
 GC_SRCS += gc-stock gc-debug gc-pages gc-page-profiler
@@ -67,7 +67,7 @@ CG_LLVMLINK :=
 ifeq ($(JULIACODEGEN),LLVM)
 # Currently these files are used by both GCs. But we should make the list specific to stock, and MMTk should have its own implementation.
 GC_CODEGEN_SRCS := llvm-final-gc-lowering llvm-late-gc-lowering llvm-gc-invariant-verifier
-ifneq (${MMTK_PLAN},None)
+ifeq (${USE_THIRD_PARTY_GC},mmtk)
 FLAGS += -I$(MMTK_API_INC)
 GC_CODEGEN_SRCS += llvm-late-gc-lowering-mmtk
 else
@@ -122,7 +122,7 @@ UV_HEADERS += uv.h
 UV_HEADERS += uv/*.h
 endif
 PUBLIC_HEADERS := $(BUILDDIR)/julia_version.h $(wildcard $(SRCDIR)/support/*.h) $(addprefix $(SRCDIR)/,work-stealing-queue.h gc-interface.h gc-tls-common.h julia.h julia_assert.h julia_threads.h julia_fasttls.h julia_locks.h julia_atomics.h jloptions.h)
-ifneq (${MMTK_PLAN},None)
+ifeq (${USE_THIRD_PARTY_GC},mmtk)
 	PUBLIC_HEADERS += $(addprefix $(SRCDIR)/,gc-tls-mmtk.h)
 else
 	PUBLIC_HEADERS += $(addprefix $(SRCDIR)/,gc-tls-stock.h)
@@ -191,8 +191,14 @@ LIBJULIA_PATH_REL := libjulia
 endif
 
 COMMON_LIBPATHS := -L$(build_libdir) -L$(build_shlibdir)
-RT_LIBS := $(WHOLE_ARCHIVE) $(LIBUV) $(WHOLE_ARCHIVE) $(LIBUTF8PROC) $(NO_WHOLE_ARCHIVE) $(LIBUNWIND) $(RT_LLVMLINK) $(OSLIBS) $(LIBTRACYCLIENT) $(LIBITTAPI) $(MMTK_LIB)
-CG_LIBS := $(LIBUNWIND) $(CG_LLVMLINK) $(OSLIBS) $(LIBTRACYCLIENT) $(LIBITTAPI) $(MMTK_LIB)
+RT_LIBS := $(WHOLE_ARCHIVE) $(LIBUV) $(WHOLE_ARCHIVE) $(LIBUTF8PROC) $(NO_WHOLE_ARCHIVE) $(LIBUNWIND) $(RT_LLVMLINK) $(OSLIBS) $(LIBTRACYCLIENT) $(LIBITTAPI)
+CG_LIBS := $(LIBUNWIND) $(CG_LLVMLINK) $(OSLIBS) $(LIBTRACYCLIENT) $(LIBITTAPI)
+
+ifeq (${USE_THIRD_PARTY_GC},mmtk)
+RT_LIBS += $(MMTK_LIB)
+CG_LIBS += $(MMTK_LIB)
+endif
+
 RT_DEBUG_LIBS := $(COMMON_LIBPATHS) $(WHOLE_ARCHIVE) $(BUILDDIR)/flisp/libflisp-debug.a $(WHOLE_ARCHIVE) $(BUILDDIR)/support/libsupport-debug.a -ljulia-debug $(RT_LIBS)
 CG_DEBUG_LIBS := $(COMMON_LIBPATHS) $(CG_LIBS) -ljulia-debug -ljulia-internal-debug
 RT_RELEASE_LIBS := $(COMMON_LIBPATHS) $(WHOLE_ARCHIVE) $(BUILDDIR)/flisp/libflisp.a $(WHOLE_ARCHIVE) $(BUILDDIR)/support/libsupport.a -ljulia $(RT_LIBS)
diff --git a/src/gc-tls-third-party.h b/src/gc-tls-third-party.h
new file mode 100644
index 0000000000000..bd7a2a76b011c
--- /dev/null
+++ b/src/gc-tls-third-party.h
@@ -0,0 +1,12 @@
+// This file is a part of Julia. License is MIT: https://julialang.org/license
+
+// Pick the appropriate third-party implementation
+#ifdef WITH_THIRD_PARTY_HEAP
+#if WITH_THIRD_PARTY_HEAP == 1
+#include "gc-tls-mmtk.h"
+#endif
+// To be extended by other GC implementations
+// #if WITH_THIRD_PARTY_HEAP == X
+//      #include gc-tls-XXXX.h
+// #endif
+#endif
\ No newline at end of file
diff --git a/src/julia_threads.h b/src/julia_threads.h
index 061eb9266e7a7..b422942d52c8d 100644
--- a/src/julia_threads.h
+++ b/src/julia_threads.h
@@ -4,10 +4,10 @@
 #ifndef JL_THREADS_H
 #define JL_THREADS_H
 
-#ifndef MMTK_GC
+#ifndef WITH_THIRD_PARTY_HEAP
 #include "gc-tls-stock.h"
 #else
-#include "gc-tls-mmtk.h"
+#include "gc-tls-third-party.h"
 #endif
 #include "gc-tls-common.h"
 #include "julia_atomics.h"