ui/vnc: VNC requires PIXMAN

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
diff --git a/meson.build b/meson.build
index 505cc59..3e60f42 100644
--- a/meson.build
+++ b/meson.build
@@ -1560,7 +1560,11 @@
 vnc = not_found
 jpeg = not_found
 sasl = not_found
-if get_option('vnc').allowed() and have_system
+if get_option('vnc') \
+             .disable_auto_if(not have_system) \
+             .require(pixman.found(),
+                      error_message: 'cannot enable VNC if pixman is not available') \
+             .allowed()
   vnc = declare_dependency() # dummy dependency
   jpeg = dependency('libjpeg', required: get_option('vnc_jpeg'),
                     method: 'pkg-config')