compiler compatibility on < 10.6

[Ken Cunningham]

> 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?
> 

Jeez.

I made them all work.

Ken