meson: Allow making subproject optional

meson.build

@@ -7,24 +7,26 @@ dependency('libavutil')
 dependency('srt')
 
 subprojects = [
-  'orc',
-  'gstreamer',
-  'libnice',
-  'gst-plugins-base',
-  'gst-plugins-good',
-  'gst-plugins-bad',
-  'gst-plugins-ugly',
-  'gst-libav',
-  'gstreamer-vaapi',
+  ['orc',              {'required': true}],
+  ['gstreamer',        {'required': true}],
+  ['libnice',          {'required': true}],
+  ['gst-plugins-base', {'required': true}],
+  ['gst-plugins-good', {'required': true}],
+  ['gst-plugins-bad',  {'required': true}],
+  ['gst-plugins-ugly', {'required': true}],
+  ['gst-libav',        {'required': true}],
+  ['gstreamer-vaapi',  {'required': false}],
 ]
 
 common_opt = ['introspection=disabled', 'examples=disabled', 'doc=disabled', 'gtk_doc=disabled']
 build_opt = ['python=disabled', 'devtools=disabled', 'ges=disabled', 'rtsp_server=disabled']
 
 run_command('meson', 'subprojects', 'download', 'gst-build')
-foreach p : subprojects
-  run_command('meson', 'wrap', 'promote', 'subprojects/gst-build/subprojects/' + p + '.wrap')
-  subproject(p, default_options: common_opt)
+foreach sp : subprojects
+  sp_name = sp[0]
+  sp_required = sp[1]['required']
+  run_command('meson', 'wrap', 'promote', 'subprojects/gst-build/subprojects/' + sp_name + '.wrap')
+  subproject(sp_name, default_options: common_opt, required: sp_required)
 endforeach
 subproject('gst-build', default_options: common_opt + build_opt)