Use ALL_CPPFLAGS, ALL_CFLAGS, ALL_CXXFLAGS and ALL_LDFLAGS internally so CFLAGS etc can be overridden on the command line.
Merged in bitshifter/premake-stable-env-flags-makefile-fix (pull request #4)

Use ALL_CPPFLAGS, ALL_CFLAGS, ALL_CXXFLAGS and ALL_LDFLAGS internally so CFLAGS etc can be overridden on the command line.