gtk-doc/: As our docs are versioned, we need to patch the index.sgml file to have correct paths there, unless we also...

Original commit message from CVS:
* gtk-doc-plugins.mak:
* gtk-doc.mak:
As our docs are versioned, we need to patch the index.sgml file to have
correct paths there, unless we also want to fork gtk-doc's xsl (which
we don't). This hopefully fixes xrefs between modules.