build:  Use AX for pkg config

Use an autoconf archive macro for pkg config so autoconf can fill
out Requires.private in sdbusplus.pc.in.

Change-Id: I62fd658b813ecde437756aefcee91ba42a3a94b2
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/configure.ac b/configure.ac
index 0a06ff0..f278240 100644
--- a/configure.ac
+++ b/configure.ac
@@ -26,7 +26,12 @@
 AM_CONDITIONAL([WANT_SDBUSPP], [test "x$enable_sdbuspp" != "xno"])
 
 # setup libsdbusplus
-PKG_CHECK_MODULES([SYSTEMD], [libsystemd >= 221], [have_systemd=yes], [have_systemd=no])
+AX_PKG_CHECK_MODULES(
+    [SYSTEMD],
+    [libsystemd >= 221],
+    [],
+    [have_systemd=yes],
+    [have_systemd=no])
 AC_ARG_ENABLE([libsdbusplus],
     AS_HELP_STRING([--disable-libsdbusplus], [Disable libsdbusplus]))