Add support for clang instead of gcc

For now I'm leaving gcc as the default, although that might change.
clang has the benefit of not requiring people to build a gcc
cross-compiler, while gcc has the benefit of not requiring people to
install clang. Hmm... Although since they have to download the gcc
source anyway, this probably isn't that much of a benefit. I'll probably
switch at some point, just not right now.
Added instructions for building with clang

I'm not particularly satisfied with the process, but it'll do for now.
It would probably be premature at this point to spend much time on
making it trivial to switch - especially since I'm not planning on
testing both compilers.