Append to ERLC_FLAGS with override for autodetects

Use

  override ERLC_FLAGS += -D<...>

instead of this (without override)

  ERLC_FLAGS += -D<...>

...but do it only for features that are auto-detected from what the
current Erlang supports. These defintions are often used to indicate
that some feature is missing, for example when using an older
version of Erlang.

The "override" makes it possible to specify additional ERLC_FLAGS on
the command line and still get any auto-detected definitions, like
this:

   make ERLC_FLAGS=-Dmy_flag=x

and (on an older Erlang) it might then compile with:

   erlc -Dmy_flag=x -DNO_HAVE_ERL20_STR_FUNCTIONS=true gpb_some_file.erl

Author: tomas-abrahamsson

Makefile

@@ -96,7 +96,8 @@ endif
 endif
 
 
-ERLC_FLAGS += -Wall +debug_info -I$(incdir)
+ERLC_FLAGS ?= -Wall +debug_info
+override ERLC_FLAGS += -I$(incdir)
 
 ERL_BATCH_FLAGS = +B -noshell -noinput
 
@@ -131,7 +132,7 @@ plt = $(GPB_PREFIX).gpb-$(OTP_MAJOR_MINOR).plt
 
 
 ifdef NO_HAVE_MAPS
-ERLC_FLAGS += -DNO_HAVE_MAPS=true
+override ERLC_FLAGS += -DNO_HAVE_MAPS=true
 else
 ## attempt to auto-detect
 ERLVM_SUPPORTS_MAPS := $(shell $(ERL) $(ERL_BATCH_FLAGS) -eval ' \
@@ -142,12 +143,12 @@ ERLVM_SUPPORTS_MAPS := $(shell $(ERL) $(ERL_BATCH_FLAGS) -eval ' \
                              receive after 10 -> ok end.' \
                          -s erlang halt)
 ifeq ($(ERLVM_SUPPORTS_MAPS),false)
-ERLC_FLAGS += -DNO_HAVE_MAPS=true
+override ERLC_FLAGS += -DNO_HAVE_MAPS=true
 endif
 endif
 
 ifdef NO_HAVE_RAND
-ERLC_FLAGS += -DNO_HAVE_RAND=true
+override ERLC_FLAGS += -DNO_HAVE_RAND=true
 else
 ## attempt to auto-detect
 ERL_HAS_RAND := $(shell $(ERL) $(ERL_BATCH_FLAGS) -eval ' \
@@ -158,12 +159,12 @@ ERL_HAS_RAND := $(shell $(ERL) $(ERL_BATCH_FLAGS) -eval ' \
                              receive after 10 -> ok end.' \
                          -s erlang halt)
 ifeq ($(ERL_HAS_RAND),false)
-ERLC_FLAGS += -DNO_HAVE_RAND=true
+override ERLC_FLAGS += -DNO_HAVE_RAND=true
 endif
 endif
 
 ifdef NO_HAVE_ERL20_STR_FUNCTIONS
-ERLC_FLAGS += -DNO_HAVE_ERL20_STR_FUNCTIONS=true
+override ERLC_FLAGS += -DNO_HAVE_ERL20_STR_FUNCTIONS=true
 else
 ## attempt to auto-detect
 ERL_HAS_ERL20_STR_FUNCTIONS := $(shell $(ERL) $(ERL_BATCH_FLAGS) -eval ' \
@@ -174,7 +175,7 @@ ERL_HAS_ERL20_STR_FUNCTIONS := $(shell $(ERL) $(ERL_BATCH_FLAGS) -eval ' \
                              receive after 10 -> ok end.' \
                          -s erlang halt)
 ifeq ($(ERL_HAS_ERL20_STR_FUNCTIONS),false)
-ERLC_FLAGS += -DNO_HAVE_ERL20_STR_FUNCTIONS=true
+override ERLC_FLAGS += -DNO_HAVE_ERL20_STR_FUNCTIONS=true
 endif
 endif