systemd: use pkg-config to determine systemd library availability

systemd: use pkg-config to determine systemd library availability

AC_CHECK_LIB fails on Debian Jessie since the ld flag it generates is
incorrect, even in the event systemd library is available.  Use
PKG_CHECK_MODULES instead.

Tested on Debian Jessie and Arch Linux.

Reported-by: Mark Pryor <tlviewer@yahoo.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Luis R. Rodriguez <mcgrof@do-not-panic.com>
Cc: Mark Pryor <tlviewer@yahoo.com>
[ ijc -- reran autogen.sh as requested ]