xfree86: link modules against Xorg symbols on Cygwin

As a PE platform, all symbols in both EXEs and DLLs must be resolved
at link time.  As Xorg modules depend on symbols in the Xorg
executable, we must build Xorg before its modules, creating an implib
from the former which is used to link the latter.  This implib must
then be installed in order to build the drivers.

Currently only two drivers are supported on Cygwin: xf86-video-dummy
(to replace Xvfb/Xfake) and xf86-video-nested (to replace Xnest/Xephyr).

Signed-off-by: Yaakov Selkowitz <yselkowitz@users.sourceforge.net>
Reviewed-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Jeremy Huddleston <jeremyhu@apple.com>