This patch makes the default compiler determination more flexible

This patch makes the default compiler determination more flexible
and also takes the sys.platform name into account. This helps on
platforms where there are multiple possible compiler backends (the
one with which Python itself was compiled is preferred over others
in this case).

The patch uses this new technique to enable using cygwin compiler
per default for cygwin compiled Pythons.

Written by Marc-Andre Lemburg. Copyright assigned to Guido van Rossum.