Allow disabling of std::filesystem in CMake.

In particular, disable its usage on MinGW running on not-Windows because the version in Debian Buster has a bug.

Author: rpavlik

src/CMakeLists.txt

@@ -53,6 +53,16 @@ option(
     "Enable exception handling in the loader. Leave this on unless your standard library is built to not throw."
     ON
 )
+
+set(BUILD_WITHOUT_STD_FILESYSTEM_DEFAULT OFF)
+if(MINGW)
+    # Debian 10's MinGW has a bug in the experimental filesystem support
+    # and MinGW on Windows doesn't apparently have any?
+    set(BUILD_WITHOUT_STD_FILESYSTEM_DEFAULT ON)
+endif()
+option(BUILD_WITHOUT_STD_FILESYSTEM "Force building without std::[experimental::]filesystem."
+       ${BUILD_WITHOUT_STD_FILESYSTEM_DEFAULT})
+
 if(CMAKE_SYSTEM_NAME STREQUAL "Windows")
     option(DYNAMIC_LOADER "Build the loader as a .dll library" OFF)
 else()

src/loader/CMakeLists.txt

@@ -222,12 +222,18 @@ if(CMAKE_COMPILER_IS_GNUCC OR CMAKE_C_COMPILER_ID MATCHES "Clang")
                 -ffunction-sections
                 -fdata-sections
     )
+    if(NOT BUILD_WITHOUT_STD_FILESYSTEM)
+        target_link_libraries(openxr_loader PRIVATE stdc++fs)
+    endif()
     # For GCC version 7.1 or greater, we need to disable the implicit fallthrough warning since
     # there's no consistent way to satisfy all compilers until they all accept the C++17 standard
     if(CMAKE_COMPILER_IS_GNUCC AND NOT (CMAKE_CXX_COMPILER_VERSION LESS 7.1))
         target_compile_options(openxr_loader PRIVATE -Wimplicit-fallthrough=0)
     endif()
 endif()
+if(BUILD_WITHOUT_STD_FILESYSTEM)
+    target_compile_definitions(openxr_loader PRIVATE DISABLE_STD_FILESYSTEM)
+endif()
 
 install(
     TARGETS openxr_loader