Serious overhaul of the C compiler interface and the two classes that

implement it (so far):
  * moved filename generation methods into CCompiler base class,
    driven by data supplied by implementation classes
  * moved a bunch of common code from UnixCCompiler to convenience
    methods in CCompiler
  * overhauled MSVCCompiler's compile/link methods to look and act
    as much as possible like UnixCCompiler's, in order to regularize
    both interface and behaviour (especially by using those new
    convenience methods)