WIP: find libsystemd with autoconfig

configure.ac

@@ -246,6 +246,22 @@ AM_CONDITIONAL([HAVE_CJWT], [test "$with_cjwt" != "no"])
 AC_SUBST([CJWT_CFLAGS])
 AC_SUBST([CJWT_LIBS])
 
+dnl libsystemd
+AC_ARG_WITH([systemd],
+  AC_HELP_STRING([--with-systemd], [(Experimental) notify systemd about daemon state @<:@default=check@:>@]),
+  [],
+  [with_systemd=check])
+if test "$with_systemd" != "no"; then
+  PKG_CHECK_MODULES([SYSTEMD], [libsystemd >= 257.0],
+		    [with_systemd=yes],[with_systemd=no])
+fi
+if test "$with_systemd" = "yes"; then
+  AC_DEFINE_UNQUOTED([HAVE_SYSTEMD], 1, [whether libsystemd is available])
+fi
+AM_CONDITIONAL([HAVE_SYSTEMD], [test "$with_systemd" != "no"])
+AC_SUBST([SYSTEMD_CFLAGS])
+AC_SUBST([SYSTEMD_LIBS])
+
 dnl libcjson
 AC_ARG_WITH([cjson],
   AC_HELP_STRING([--with-cjson], [(Experimental) use cJSON to extract private claims from JWT tokens @<:@default=check@:>@]),