build: 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

@@ -61,6 +61,16 @@ 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

@@ -145,9 +145,11 @@ if(CMAKE_SYSTEM_NAME STREQUAL "Linux")
     set_target_properties(openxr_loader PROPERTIES SOVERSION "${MAJOR}" VERSION "${MAJOR}.${MINOR}.${PATCH}")
     target_link_libraries(
         openxr_loader
-        PRIVATE stdc++fs
         PUBLIC m
     )
+    if(NOT BUILD_WITHOUT_STD_FILESYSTEM)
+        target_link_libraries(openxr_loader PRIVATE stdc++fs)
+    endif()
 
     add_custom_target(
         libopenxr_loader.so.${MAJOR}.${MINOR} ALL
@@ -236,6 +238,9 @@ if(CMAKE_COMPILER_IS_GNUCC OR CMAKE_C_COMPILER_ID MATCHES "Clang")
         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