Bob Ham authored Dec 04, 2015 and Adam Jackson committed Dec 08, 2015

The configure script looks for the libsystemd-daemon pkg-config
module.  If the configure script finds it, the script will add
libsystemd-daemon to a list of modules which are used to consolidate
CFLAGS and LIBS.

The check for libsystemd-daemon was altered to fallback to libsystemd
if libsystemd-daemon was not found (libsystemd-daemon was brought into
libsystemd).  Unfortunately, the configure script still adds
"libsystemd-daemon" to the list of modules to consolidate, instead of
"libsystemd".  With this patch, we set a variable depending on which
pkg-config module is found and add that to the module list instead.

Changes since v1:
- Rearranged logic so that we do a versioned check for libsystemd
  first, then look for libsystemd-daemon.
- Cleaned up the check a bit, only performing the module checks if we
  don't have --with-systemd-daemon=no, in a similar style to
  --with-dtrace.
- Changed the variable name to LIBSYSTEMD_DAEMON as per feedback.

Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Bob Ham <bob.ham@collabora.com>