build: Check the version of GTK-Doc

We lost widget style properties and container child properties in this
new major API version; sadly, GTK-Doc still tries to inspect widgets and
containers for them. Newer version of GTK-Doc know how to do a version
check against GTK, but older versions can't know that.

We can't specify a minimum version of GTK-Doc to use with Meson, so we
need to check the version manually.

docs/reference/meson.build

@@ -1,4 +1,14 @@
-if get_option('gtk_doc')
+build_docs = get_option('gtk_doc')
+if build_docs
+  gtkdoc_scangobj = find_program('gtkdoc-scangobj')
+  gtkdoc_ver = run_command(gtkdoc_scangobj, '--version').stdout().strip()
+  if gtkdoc_ver.version_compare('<1.30')
+    warning('GTK-Doc version is too old for GTK4')
+    build_docs = false
+  endif
+endif
+
+if build_docs
   glib_prefix = dependency('glib-2.0').get_pkgconfig_variable('prefix')
   glib_docpath = join_paths(glib_prefix, 'share', 'gtk-doc', 'html')
 
@@ -15,8 +25,8 @@ if get_option('gtk_doc')
 
   src_dir_conf = configuration_data()
   src_dir_conf.set('SRC_DIR', meson.source_root())
-endif
 
   subdir('gdk')
   subdir('gsk')
   subdir('gtk')
+endif