Treat .m files consistently as C across all generators

Objective C sources should be compiled with the C compiler, not C++.

The Xcode generator correctly classifies ".m" sources already.  The
cmSystemTools::GetFileFormat method was fixed by commit v2.8.0~1782 (fix
for 7045, use gcc for .m, 2008-08-19) but it is not used by any of the
generator since commit v2.4.0~2819 (major changes to support addition of
languages, 2004-09-22).

Fix the CMAKE_<LANG>_SOURCE_FILE_EXTENSIONS table entries so that
cmGlobalGenerator::GetLanguageFromExtension tells the Makefile, Ninja,
and VS IDE generators to compile ".m" sources as C.  This makes behavior
consistent on all generators.

Signed-off-by: Tim Blechmann <[email protected]>

Modules/CMakeCCompiler.cmake.in

@@ -26,7 +26,7 @@ if(CMAKE_COMPILER_IS_MINGW)
   set(MINGW 1)
 endif()
 set(CMAKE_C_COMPILER_ID_RUN 1)
-set(CMAKE_C_SOURCE_FILE_EXTENSIONS c)
+set(CMAKE_C_SOURCE_FILE_EXTENSIONS c;m)
 set(CMAKE_C_IGNORE_EXTENSIONS h;H;o;O;obj;OBJ;def;DEF;rc;RC)
 set(CMAKE_C_LINKER_PREFERENCE 10)
 

Modules/CMakeCXXCompiler.cmake.in

@@ -27,7 +27,7 @@ if(CMAKE_COMPILER_IS_MINGW)
 endif()
 set(CMAKE_CXX_COMPILER_ID_RUN 1)
 set(CMAKE_CXX_IGNORE_EXTENSIONS inl;h;hpp;HPP;H;o;O;obj;OBJ;def;DEF;rc;RC)
-set(CMAKE_CXX_SOURCE_FILE_EXTENSIONS C;M;c++;cc;cpp;cxx;m;mm;CPP)
+set(CMAKE_CXX_SOURCE_FILE_EXTENSIONS C;M;c++;cc;cpp;cxx;mm;CPP)
 set(CMAKE_CXX_LINKER_PREFERENCE 30)
 set(CMAKE_CXX_LINKER_PREFERENCE_PROPAGATES 1)