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 <tim@klingt.org>
Merge topic 'compile-m-as-c' into next

95cdf132 Treat .m files consistently as C across all generators