[v3] makefile tweaks for CXXFLAGS, etc

[Benjamin Kosnik]

CXXCOMPILE and LTCXXCOMPILE now treat CXXFLAGS the same. This makes
certain rebuilding options easier, for instance if you try things like
"-O3 -fimplicit-templates" on new files you get consistent behavior.

The rest are just cleanups for c++config inclusion. I started out doing
this to use a macro hack memory debugging wrapper I put in c++config. 

tested x86/linux

-benjamin