[build] fix sbd build when pacemaker is not installed on standard paths

how to reproduce:

• remove all pacemaker installations (either rpms or manual)
• install pacemaker with: ./configure --prefix=/srv/foo --exec-prefix=/srv/foo make make install
• on sbd: autoreconf -v -i ./configure PKG_CONFIG_PATH=/srv/foo

sbd will fail to find pacemaker headers and libraries because the
current configure assumes that they are located in standard paths.

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