doc: fix build with latest meson

doc: fix build with latest meson

The latest versions of meson don't build targets when build_by_default is
false but install is true, unlike older versions. We can fix this by having
both build_by_default and install settings come from the build-time option.

Bugzilla ID: 303
Fixes: d02a2dab2dfb ("doc: support building HTML guides with meson")
Fixes: 720b14db3ae2 ("build: generate API documentation with meson")
Cc: stable@dpdk.org
Reported-by: Ali Alnubani <alialnu@mellanox.com>
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Luca Boccassi <bluca@debian.org>