configure: Don't require pthread-stubs on Cygwin

Commit 1f4869a2 unconditionally requires pthread-stubs.  Unfortunately, the
cleverness that pthread-stubs is doesn't work with PE/COFF, and historically
Cygwin doesn't have a pthread-stubs.pc.

Don't require pthread-stubs on Cygwin.

Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>
1 file changed