examples/gl/gtk: Fix build on macOS

The gdkquartz.h include pulls into Apple Obj-C frameworks so the compiler needs
to handle the gstgtkhelper library as such.

Fixes #518

tests/examples/gl/gtk/meson.build

@@ -3,9 +3,15 @@ if cc.has_argument ('-Wno-parentheses')
   extra_args += '-Wno-parentheses'
 endif
 
+if host_system == 'darwin'
+  extra_c_args = ['-xobjective-c']
+else
+  extra_c_args = []
+endif
+
 gstgtkhelper = static_library ('gstgtkhelper',
   ['gstgtk.c'],
-  c_args : gst_plugins_base_args,
+  c_args : gst_plugins_base_args + extra_c_args,
   include_directories : [configinc, libsinc],
   dependencies : [gst_base_dep, video_dep, gtk_dep],
   install : false)

tests/examples/gl/meson.build

@@ -1,9 +1,5 @@
 subdir('cocoa')
 subdir('generic')
-# FIXME: gtk is temporarily disabled. 
-#        See. https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/issues/518
-if host_system != 'darwin'
 subdir('gtk', if_found : gtk_dep)
-endif
 # FIXME: subdir('qt')
 subdir('sdl')