Enables fully dynamic string in GCC 4.7

The default Mac OSX compilers enable fully dynamic strings (http://newartisans.com/2009/10/a-c-gotcha-on-snow-leopard/). Without this option, libraries compiled with Homebrew GCC will often fail when linked against system libraries. Adding '--enable-fully-dynamic-string' prevents this.

This is a larger question than just for GCC 4.7, really, but I thought I'd start with what I use.