- added big comment describing possible problems
  - look for and react to versions of gcc, ld, and dlltool; mainly
    this is done by the 'get_versions()' function and the CygwinCCompiler
    and Mingw32CCompiler constructors
  - move 'check_config_h()' to end of file and defer calling it until
    we need to (ie. in the CygwinCCompiler constructor)
  - lots of changes in 'link_shared_object()' -- mostly seems to be
    library and DLL stuff, but I don't follow it entirely