Extend compiler() method with optional depends argument.

This change is not backwards compatible.  If a compiler subclass
exists outside the distutils package, it may get called with the
unexpected keyword arg.  It's easy to extend that compiler by having
it ignore the argument, and not much harder to do the right thing.  If
this ends up being burdensome, we can change it before 2.3 final to
work harder at compatibility.

Also add _setup_compile() and _get_cc_args() helper functions that
factor out much of the boilerplate for each concrete compiler class.