macports-gcc-XX and macports-clang-XX

[Jeremy Huddleston]

Rather than hardcode every version of gcc and clang into base, we should support these configure.compiler options dynamically.  My tcl-foo isn't quite good enough to easily do that, but hopefully somebody has the magic necessary.  I just thought I'd throw it out there since the list of compilers will only grow over the coming years, and it sucks to have to wait for a new version of base to depend on a new compiler "the right way".

--Jeremy