Feature incremental build

This adds incremental build support for easy debugging of compile issues. This builds upon Thomas' static gcc build and also fixes static building to work on Ubuntu 13.04 (it was a change to use --enable-static in mpfr and mpc).

I have a funny OS X machine that only has clang, the driving motivation for this, which was able to successfully to build the toolchain after making a one line modification:
- https://github.com/ndreys/gdb/blob/master/sim/arm/armsupp.c#L639
<br> gdb has a function that is missing a return value in clang's eyes.

This works well enough, and you can use ccache to fill in the gaps if for some reason something rebuilds.