build:  Use AX for pkg config

Use an autoconf archive macro for pkg config so autoconf can fill
out Requires.private in phosphor-dbus-interfaces.pc.in.

Change-Id: Ica803a0f758871b192502be5c2cdc6e166337df2
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/configure.ac b/configure.ac
index 64c28bc..ee42dbe 100644
--- a/configure.ac
+++ b/configure.ac
@@ -28,8 +28,8 @@
         AC_MSG_ERROR([Cannot find sdbus++]))
 
     # Checks for libraries.
-    PKG_CHECK_MODULES([SYSTEMD], [libsystemd >= 221],,
-        [AC_MSG_ERROR(["systemd required and not found."])])
+    AX_PKG_CHECK_MODULES([SYSTEMD], [], [libsystemd >= 221], [], [AC_MSG_ERROR(["systemd required and not found."])])
+    AX_PKG_CHECK_MODULES([SDBUSPLUS], [], [sdbusplus], [], [AC_MSG_ERROR(["sdbusplus required and not found."])])
 
     # Checks for typedefs, structures, and compiler characteristics.
     AX_CXX_COMPILE_STDCXX_14([noext])