configure: use resource-agents pkg-config info to determine ocfdir

and allow manual override --with-ocfdir.

Recent version of resource-agents ship resource-agents.pc, containing
build information on ocfrootdir. Use that as authoritative information.

pacemaker and other projects have already switched to use it.

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>