ui/spice: SPICE/QXL 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 3e60f42..0f578dd 100644
--- a/meson.build
+++ b/meson.build
@@ -1015,7 +1015,11 @@
                               method: 'pkg-config')
 endif
 spice = not_found
-if not get_option('spice').auto() or have_system
+if get_option('spice') \
+             .disable_auto_if(not have_system) \
+             .require(pixman.found(),
+                      error_message: 'cannot enable SPICE if pixman is not available') \
+             .allowed()
   spice = dependency('spice-server', version: '>=0.14.0',
                      required: get_option('spice'),
                      method: 'pkg-config')