build: use pkg-config way to detect and use pacemaker header files

If pacemaker is built/installed in non-standard location, the current
checks and includes will fail.

Also, pay the respective attention to .travis.yml.

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