systemd: fix detection of libsystemd

systemd: fix detection of libsystemd

On Fedora 25 detection of libsystemd actually fails due to wrong
assumptions in the configure test. conf.CHECK_LIB returns a list
so 'not conf.CHECK_LIB(...)' is always False and we never get to check
libsystemd.

Instead, remember result of checking pkg-config for separate
libsystemd-daemon and libsystemd-journal libraries. If they miss,
attempt to use libsystemd library instead.

Signed-off-by: Alexander Bokovoy <ab@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Tue May  2 13:05:43 CEST 2017 on sn-devel-144

(cherry picked from commit 09bc5b5374227a555f580c3d7c1d82f15bb818bc)

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12764
systemd: fix detection of libsystemd

Autobuild-User(v4-6-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-6-test): Wed May  3 20:43:57 CEST 2017 on sn-devel-144