glade: Verify catalog data via xmllint

data/meson.build

+glade_xml = 'glade/libhandy.xml'
+
 if get_option('glade_catalog')
+  gladeui = dependency('gladeui-2.0')
+  dtd = join_paths(gladeui.get_pkgconfig_variable('catalogdir'), 'glade-catalog.dtd')
+
   glade_catalogdir = join_paths(get_option('prefix'), get_option('datadir'), 'glade', 'catalogs')
-  install_data('glade/libhandy.xml', install_dir: glade_catalogdir)
+  install_data(glade_xml, install_dir: glade_catalogdir)
+
+  xmllint = find_program('xmllint', required: true)
+  if xmllint.found()
+    custom_target(
+      'xmllint',
+        build_by_default: true,
+        input: glade_xml,
+        output: 'doesnotexist',
+        command: [xmllint, '--dtdvalid', dtd, '--noout', '@INPUT@'],
+    )
+  endif
 endif