Support Clang as a `cross-tool' and use it under Android.

Support Clang as a `cross-tool' and use it under Android.

Add the necessary machinery to use Clang instead.  Annoyingly, the
Android NDK doesn't name its Clang wrappers with the usual GNU
architecture triples, so there's an exciting amount of hacking to be
done.

What I don't have yet is a principled way to configure which compiler
should get chosen for a given build.