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>

configure.ac

@@ -735,6 +735,10 @@ gnu*)
     dnl Disable by default on GNU/Hurd
     driglx_direct_default="no"
     ;;
+cygwin*)
+    dnl Disable by default on cygwin
+    driglx_direct_default="no"
+    ;;
 *)
     driglx_direct_default="yes"
     ;;
@@ -1022,6 +1026,16 @@ if test "$mesa_driver" = dri -o "$mesa_driver" = no; then
             DEFINES="$DEFINES -DGLX_DIRECT_RENDERING"
         fi
         ;;
+    cygwin*)
+        DEFINES="$DEFINES -DUSE_EXTERNAL_DXTN_LIB=1 -DIN_DRI_DRIVER"
+        DEFINES="$DEFINES -DGLX_INDIRECT_RENDERING"
+        if test "x$driglx_direct" = xyes; then
+            DEFINES="$DEFINES -DGLX_DIRECT_RENDERING"
+        fi
+        if test "x$DRI_DIRS" = "xyes"; then
+            DRI_DIRS="swrast"
+        fi
+        ;;
     esac
 
     # default drivers