* Makefile.am (path-check): Make sure distdir is writeable before

	* Makefile.am (path-check): Make sure distdir is writeable before
	removing it.
	* automake.in (handle_texinfo): Added `cm' and `ov' extensions.
	(handle_dist_worker): Make sure distdir is writeable before
	removing it.
	(initialize_global_constants [$dist_trailer]): Make sure distdir
	is writeable before removing it.
	* dist.am (distcheck): Make the new source tree read-only before
	compiling.  Idea from Karl Berry.