Fix build on Cygwin by ensuring WIN32 check triggers only on MinGW

The type of fd_mask was changed in Cygwin 2.4.0 headers from 'long' to
'unsigned long'.  This exposes an existing problem with winauth.c, which
includes Xwindows.h (which includes windows.h, which defines WIN32),
before including osdep.h, which causes the now conflicting definition of
fd_mask in osdep.h to be exposed:

In file included from ../os/osdep.h:198:18: error: conflicting types for
‘fd_mask’ typedef long int fd_mask; /usr/include/sys/select.h:46:23:
note: previous declaration of ‘fd_mask’ was here typedef unsigned long
fd_mask;

Adjust the include guards in osdep.h to make sure we only use WIN32
guarded code when not compiling for Cygwin (i.e. WIN32 && !__CYGWIN__)

This isn't a very elegant, but unfortunately appears to be the best
solution, since it doesn't seem to be possible to write the test in a
positive form.

Future work: Should also audit of all the other uses of WIN32 in
xserver, and make sure they are correct.

Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>
(cherry picked from commit 30b7d7995e)

os/osdep.h

@@ -72,7 +72,7 @@ SOFTWARE.
 #if defined(NOFILE) && !defined(NOFILES_MAX)
 #define OPEN_MAX NOFILE
 #else
-#if !defined(WIN32)
+#if !defined(WIN32) || defined(__CYGWIN__)
 #define OPEN_MAX NOFILES_MAX
 #else
 #define OPEN_MAX 512
@@ -171,7 +171,7 @@ extern fd_set ClientsWriteBlocked;
 extern fd_set OutputPending;
 extern fd_set IgnoredClientsWithInput;
 
-#ifndef WIN32
+#if !defined(WIN32) || defined(__CYGWIN__)
 extern int *ConnectionTranslation;
 #else
 extern int GetConnectionTranslation(int conn);
@@ -185,7 +185,7 @@ extern Bool AnyClientsWriteBlocked;
 extern WorkQueuePtr workQueue;
 
 /* in WaitFor.c */
-#ifdef WIN32
+#if defined(WIN32) && !defined(__CYGWIN__)
 typedef long int fd_mask;
 #endif
 #define ffs mffs