auxiliary/vl: Build vl_winsys_dri.c only when needed.

With commit c39dbfdd0f7(auxiliary/vl: bring back the VL code for the dri
targets) we did not fully consider users of dri-swrast alone. Thus we
ended up trying to compile the dri2 specific code on platform which lack
it - Cygwin for example.

Cc: "10.5" <mesa-stable@lists.freedesktop.org>
Reported-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Jon TURNEY <jon.turney@dronecode.org.uk>
diff --git a/configure.ac b/configure.ac
index 21e396e..beb7a7d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1577,11 +1577,19 @@
         "x$enable_vdpau" = xyes -o \
         "x$enable_omx" = xyes -o \
         "x$enable_va" = xyes; then
-    PKG_CHECK_MODULES([VL], [x11-xcb xcb xcb-dri2 >= $XCBDRI2_REQUIRED])
     need_gallium_vl=yes
 fi
 AM_CONDITIONAL(NEED_GALLIUM_VL, test "x$need_gallium_vl" = xyes)
 
+if test "x$enable_xvmc" = xyes -o \
+        "x$enable_vdpau" = xyes -o \
+        "x$enable_omx" = xyes -o \
+        "x$enable_va" = xyes; then
+    PKG_CHECK_MODULES([VL], [x11-xcb xcb xcb-dri2 >= $XCBDRI2_REQUIRED])
+    need_gallium_vl_winsys=yes
+fi
+AM_CONDITIONAL(NEED_GALLIUM_VL_WINSYS, test "x$need_gallium_vl_winsys" = xyes)
+
 if test "x$enable_xvmc" = xyes; then
     PKG_CHECK_MODULES([XVMC], [xvmc >= $XVMC_REQUIRED])
     enable_gallium_loader=$enable_shared_pipe_drivers