Make the install-html target of apidoc/manual be conditional

Make the install-html target of apidoc/manual be conditional

When running 'make distcheck', make install is automatically run,
which runs 'make install-html', especially in the apidoc and manual
sub-directories.  That triggers the building of the documentation,
using doxygen and python-sphinx even when these two packages are not
installed; in this case, install-html fails because doxygen or or the
python-sphinx binary is not found.  So this patch conditionnaly make the
install-html target run.  To run them, one must enable the building of
the apidoc or the manuals.  Note that just installing doxygen or
python-sphinx enables the building of the apidoc and the manuals.

	* configure.ac: add --enable-apidoc and --enable-manual.  Add the
	two ENABLE_APIDOC and ENABLE_MANUAL automake conditional
	variables.  Add the activation of the apidoc and manual into the
	final package configuration report.
	* doc/Makefile.am: Make the install-html, install-data-local and
	uninstall-local targets conditional on the ENABLE_APIDOC
	conditional variable.
	* doc/manuals/Makefile.am: Likewise, make the install-html,
	install-data-local and uninstall-data-local conditional on the
	ENABLE_MANUAL conditional variable.

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