Makefiles: use appropriate preprocessor vs. compiler flags

We should differentiate preprocessor vs. C++ compiler vs. C compiler
flags, according to the CPPFLAGS, CXXFLAGS, and CFLAGS conventions. That
way we can, for instance, easily add the appropriate --std=XXX flags
necessary for compiling with a C++11 (or C++0x) or C11 (or C1x)
compliant compiler.