Fix manual documentation generation

Fix manual documentation generation

The manual building was failing in make distcheck because it was being
built using the standard 'html' target.  That target needs the output
to not be rebuilt each time make install-html is called.  Which is not
easily possible for us.  So for now, the apidoc is being generated
using the html-doc target.  Also fix some other issues in the Makefile.

	* doc/manuals/Makefile.am: Trigger the manual generation from the
	html-doc target.  Add the source files to the distribution.  Fix
	the clean target.

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