[gtk] build: Error if no backends enabled

[Patrick Griffis <pgriffis src gnome org>]

commit 971f46e1f292120322143d23201d8d377e225354
Author: Patrick Griffis <tingping tingping se>
Date:   Tue Feb 27 13:42:28 2018 -0500

    build: Error if no backends enabled

 gdk/meson.build | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/gdk/meson.build b/gdk/meson.build
index 267f53fd27..466e11ff41 100644
--- a/gdk/meson.build
+++ b/gdk/meson.build
@@ -204,6 +204,10 @@ foreach backend : ['broadway', 'quartz', 'wayland', 'win32', 'x11']
   endif
 endforeach
 
+if gdk_backends.length() == 0
+  error('No backends enabled')
+endif
+
 # FIXME: might have to add '-xobjective-c' to c_args for quartz backend?
 libgdk = static_library('gdk',
   sources: [gdk_sources, gdk_backends_gen_headers, gdkconfig],