Generate the manual html documentation in the proper build directory

Generate the manual html documentation in the proper build directory

	* doc/manuals/Makefile.am: Renamed doc/manuals/Makefile into this.
	(BUILDDIR): Make this variable point to $(builddir)/_build as
	opposed to just _build previously.
	(SOURCEDIR): New variable that points to the source dir as known
	by the autotools magic.
	(ALLSPHINXOPTS): Make the source dir refer to the new
	$(SOURCEDIR), rather than just '.' as previously.
	* configure.ac (doc/manuals/Makefile): Generate this now.
	* doc/Makefile.am: Link the sub-directory doc/manuals.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>