doc: Adapt GLib documentation path to GLib 2.79.0

GLib 2.79.0 switched from gtk-doc to gi-docgen, whose format and
installation location are both not compatible. The libmodulemd build
script reported an error because of the new location.

This patch fixes the build failure by using the new location if GLib
is 2.79.0 or greater.

Although the libmodulemd documentation references to GLib remain
unresolved now (i.e. names of the functions and the types are not
hyperlinks), we keep using the new location because the new GLib
*.devhelp2 indices remain there and they can only improve over the
time.

If this expection does not fulfill, libmodulemd will either migrate to
gi-docgen, or drop the hard build-time dependency on GLib
documentation.

Reimplements: #611

meson.build

@@ -70,7 +70,13 @@ with_docs = get_option('with_docs')
 gtk_doc_referred_paths = []
 if with_docs
   gtkdoc = dependency('gtk-doc')
-  glib_docpath = join_paths(glib_prefix, 'share', 'gtk-doc', 'html')
+
+  if glib.version().version_compare('<2.79.0')
+    glib_docpath = join_paths(glib_prefix, 'share', 'gtk-doc', 'html')
+  else
+    glib_docpath = join_paths(glib_prefix, 'share', 'doc', 'glib-2.0')
+    warning('glib >= 2.79.0 documention might not be properly referred from libmodulemd documentation.')
+  endif
 
   foreach referred_module : [ 'glib', 'gobject' ]
     doc_module_path = join_paths(glib_docpath, referred_module)