[GTK] doc rebasing does not respect DESTDIR

[GTK] doc rebasing does not respect DESTDIR
https://bugs.webkit.org/show_bug.cgi?id=78177

Reviewed by Martin Robinson.

* GNUmakefile.am: Pass DESTDIR on to generate-gtkdoc, when
calling it for rebasing.
* gtk/generate-gtkdoc:
(get_common_options): Handle the new --virtual-root option.
* gtk/gtkdoc.py:
(GTKDoc.rebase_installed_docs): If a virtual-root has been given, pass
it on to gtkdoc-rebase as dest-dir, and prefix the htmldir with it.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@107164 268f45cc-cd09-0410-ab3c-d52691b4dbfc