Fix TOC generation for build dir != src dir

Fix TOC generation for build dir != src dir
	* lib/doc/depend.py
	(documents): Add dir_prefix argument
	(all_documents): Ditto
	(main): Create document list with empty prefix, since the prefix will
	be addded by a Makefile variable
	(main): Don't use isdir to detect english language, since that does
	not work for build dir != src dir

	* lib/doc/doc_toc.py
	(main): Don't use isdir to detect english language, since that does
	not work for build dir != src dir, rather create the dir if it does
	not exists.

	* lib/doc/Makefile.depend: Regenerated

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@17469 a592a061-630c-0410-9148-cb99ea01b6c8