Have TraceMonkey prefer the compiler that built Ruby

[Our version of] TraceMonkey won't build with clang. But then, nor will
Ruby 1.8.7, so there's a moderate chance it can tell us where to find a
"better" compiler.

This obviously doesn't help you if you're using a remotely modern Ruby.
The next best choice would seem to be that we go spelunking for a
suitable compiler by ourselves. :/

Also, have TraceMonkey skip its tests... there was another compile error
in there.