- The source-file signature of try_compile looks up the language
    of the source file using the extension-to-language map so that
    it knows what language to enable in the generated project.
  - This map needs to be filled before loading a file specified by
      CMAKE_USER_MAKE_RULES_OVERRIDE
      CMAKE_USER_MAKE_RULES_OVERRIDE_<LANG>
    so that the user file may call the try_compile() source-file
    signature.
  - It must still be re-filled after loading CMake<LANG>Information.cmake
    in case the compiler- or platform-specific files added anything.
  - See bug #7340.