Improvements to cross.sh

[singerng]

Any particular reason it can't automagically download and unpack the source (GCC and binutils)? Also, I always install to /usr/local/cross. There should be a way to modify the PREFIX variable without touching the script. As such I'll open this issue so we can talk about it and somebody can fix it.

[singerng]

Even better, it should be able to detect if the three major dependencies for GCC (GMP, MPFR, MPC), and download/build them if they are not present. Also, we should probably test this script on Linux, OS X, and Windows.