Disable direct rendering on Cygwin

Disable direct rendering on Cygwin

Add Cygwin platform-specific settings and drivers to build for dri driver:
- by default, disable direct rendering.
- if direct rendering is enabled, the swrast dridriver is the only one it's
sensible to try to build (this doesn't work at the moment as additional patches
are required to build a libGL which can load just swrast without the DRM headers,
even though there's no actual functional dependency)

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Julien Cristau <jcristau@debian.org>