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/Makefile.am b/Makefile.am
index 464c668..6927ce4 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -5,8 +5,8 @@
 
 libphosphor_dbusdir = ${libdir}
 libphosphor_dbus_LTLIBRARIES = libphosphor_dbus.la
-libphosphor_dbus_la_LDFLAGS = $(SYSTEMD_LIBS) -version-info 0:0:0 -shared
-libphosphor_dbus_la_CXXFLAGS = $(SYSTEMD_CFLAGS)
+libphosphor_dbus_la_LDFLAGS = $(SYSTEMD_LIBS) $(SDBUSPLUS_LIBS) -version-info 0:0:0 -shared
+libphosphor_dbus_la_CXXFLAGS = $(SYSTEMD_CFLAGS) $(SDBUSPLUS_CFLAGS)
 
 BUILT_SOURCES = libphosphor_dbus.cpp
 CLEANFILES = libphosphor_dbus.cpp