Use target-prefixed executables when cross-compiling

[Skirmisher]

This adds some logic to determine if cross-compilation is occurring, and picks the right prefix to use depending on the target, similar to what the cc crate does. It also reads the environment to allow the executable to be overridden by the user, rather than just the library author.

I removed the conditional .exe suffix to make things simpler, since Windows doesn't need it when running from PATH.