Changed RIOT/Makefile.include and boards/native/Makefile.include to compile C and C++ files.
Added another_c_cpp_mix and cpp_mix to examples folder.
address comment by @LudwigOrtmann - removed trailing space in RIOT/Makefile.include
address comment by @LudwigOrtmann - leave CXXUWFLAGS and CXXEXFLAGS empty in boards/native/Makefile.include