build: Do not compile GStreamer plugin if dependencies are not met

Be default compilation of GStreamer plugin is set to "auto" meaning
that should be compiled only if dependencies are met.
Make the compilation dependent on dependencies.
If the plugin is set to "enabled" dependency check will fail
if dependencies are not found.
Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Acked-by: Kevin Pouget <kpouget@redhat.com>

meson.build

@@ -63,10 +63,12 @@ compile_gst_plugin = false
 gst_deps = []
 if not get_option('gst-plugin').disabled()
   deps = ['gstreamer-1.0', 'gstreamer-app-1.0']
+  compile_gst_plugin = true
   foreach dep : deps
-    gst_deps += dependency(dep, version: '>= 1.10', required : get_option('gst-plugin'))
+    dep = dependency(dep, version: '>= 1.10', required : get_option('gst-plugin'))
+    compile_gst_plugin = compile_gst_plugin and dep.found()
+    gst_deps += dep
   endforeach
-  compile_gst_plugin = true
 endif
 
 compile_tests = false