The cygwinccompiler.get_versions() function only handles versions numbers of
the form "x.y.z".  The attached patch enhances get_versions() to handle "x.y"
too (i.e., the ".z" is optional).

This change causes the unnecessary "--entry _DllMain@12" link option to be
suppressed for recent Cygwin and Mingw toolchains. Additionally, it directs
recent Mingw toolchains to use gcc instead of dllwrap during linking.