meson: allow disabling appdata validation

data/meson.build

@@ -23,7 +23,7 @@ appstream_file = i18n.merge_file(
 )
 
 appstream_util = find_program('appstream-util', required: false)
-if appstream_util.found()
+if appstream_util.found() and get_option('validate_appdata')
   test('Validate appstream file', appstream_util,
     args: ['validate', appstream_file]
   )

meson_options.txt

@@ -1,2 +1,5 @@
 option('predefined_salt', type: 'string',
        description: 'Hexlified salt for debugging purposes')
+
+option('validate_appdata', type: 'boolean', value: true,
+       description: 'Disable validation of appdata file')