modified Makefile.include to build c and c++ in user's app
added CXX and CXXFLAGS in boards/native/Makefile.include
added test_another_mix_compiler to tests folder
added BytesGalore's test_cpp_mix to tests folder
Merge branch 'nhat_mixCompiler' of https://github.com/vanhieubk/RIOT into fix_typo_in_doxy
fixed copy paste typo in doxy comments