compiler compatibility on < 10.6

[Joshua Root]

It recently came up that base should always add the macports-gcc
compilers to the fallback list on ppc. What's less clear is what should
be done on Intel 10.5 and 10.4.

Does anyone know with certainty:

1. Which versions of macports-clang (if any) work on these platforms
without any LibcxxOnOlderSystems modifications?
2. Which versions of macports-gcc work well on these platforms?

- Josh