doc: Add gtk-doc subproject fallback

Now that we require Meson 0.52 to build the doc, we can also pull
gtk-doc as subproject when missing from the system. This requires
to pull gtk-doc master because needed changes there haven't been release
yet.

docs/reference/meson.build

@@ -50,7 +50,9 @@ if get_option('gtk_doc')
   endif
   # Check we have the minimum gtk-doc version required. Older versions won't
   # generate correct documentation.
-  dependency('gtk-doc', version : '>=1.32')
+  dependency('gtk-doc', version : '>=1.32',
+    fallback : ['gtk-doc', 'dummy_dep'],
+    default_options : ['tests=false'])
 endif
 
 subdir('gio')

subprojects/gtk-doc.wrap

@@ -0,0 +1,4 @@
+[wrap-git]
+directory=gtk-doc
+url=https://gitlab.gnome.org/GNOME/gtk-doc.git
+revision=master