Detect gtkdoc-rebase in gtk-doc.m4. Based on idea of Sven Herzberg. Fixes

	* gtk-doc.m4:
	* gtk-doc.make:
	* gtk-doc.notmpl.make:
	  Detect gtkdoc-rebase in gtk-doc.m4. Based on idea of Sven Herzberg.
	  Fixes #562310


svn path=/trunk/; revision=635