Patch #403947: On Cygwin, use the Unix compiler class, and not

the Cygwin-specific compiler class.

 (According to Jason Tishler, cygwinccompiler needs some work to
  handle the differences in Cygwin- and MSVC-Python. Makefile and
  config files are currently ignored by cygwinccompiler, as it was
  written to support cygwin for extensions which are intended to be
  used with the standard MSVC built Python.)