[gssproxy] Modernize systemd.m4 macro

libsystemd-daemon hasn't been a thing since systemd 209 was
released (February 2014, before RHEL 7). And the thing we really
want to look for is libsystemd, that's it.

Signed-off-by: David Härdeman <david@hardeman.nu>

external/systemd.m4

@@ -1,29 +1,14 @@
-dnl A macro to check presence of systemd on the system
+dnl A macro to check presence of libsystemd on the system
 AC_DEFUN([AM_CHECK_SYSTEMD],
 [
-    PKG_CHECK_EXISTS([systemd],
-                     [HAVE_SYSTEMD=yes],
-                     [HAVE_SYSTEMD=no])
-
-    dnl older system uses libsystemd
-    PKG_CHECK_EXISTS([libsystemd],
-                     [HAVE_LIBSYSTEMD=yes],
-                     [HAVE_LIBSYSTEMD=no])
-    dnl newer systemd splits libsystemd in slaler libs
-    AS_IF([test x$HAVE_LIBSYSTEMD = xyes],
-          [daemon_lib_name=libsystemd],
-          [daemon_lib_name=libsystemd-daemon])
-
-    AS_IF([test x$HAVE_SYSTEMD = xyes],
-          [PKG_CHECK_MODULES(
-              [SYSTEMD_DAEMON],
-              [$daemon_lib_name],
-              [AC_DEFINE_UNQUOTED([HAVE_SYSTEMD_DAEMON], 1,
-                                  [Build with $daemon_lib_name support])
-
-               AC_MSG_NOTICE([Will enable systemd socket activation])],
-              [AC_MSG_NOTICE([Build without $daemon_lib_name support])])],
-          [AC_MSG_NOTICE([Build without $daemon_lib_name support])])
-
-    AM_CONDITIONAL([HAVE_SYSTEMD_DAEMON], [test x"$daemon_lib_name" != x])
+    PKG_CHECK_MODULES([SYSTEMD_DAEMON],
+                      [libsystemd],
+                      [AC_DEFINE_UNQUOTED([HAVE_SYSTEMD_DAEMON], 1,
+                                          [Build with libsystemd support])
+                       HAVE_SYSTEMD_DAEMON=yes
+                       AC_MSG_NOTICE([Build with libsystemd support])],
+                      [HAVE_SYSTEMD_DAEMON=no
+                       AC_MSG_NOTICE([Build without libsystemd support])])
+
+    AM_CONDITIONAL([HAVE_SYSTEMD_DAEMON], [test x"$HAVE_SYSTEMD_DAEMON" = xyes])
 ])