Install systemd services with autotools

Change-Id: Id81ae8e2d8a2c9456ac0dac2c30c0d24dab3b694
diff --git a/Makefile.am b/Makefile.am
index 8e18167..210e499 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -5,6 +5,7 @@
 	src \
 	include \
 	sql \
+	contrib \
 	tests \
 	$(NULL)
 
@@ -12,6 +13,9 @@
 	.version \
 	$(NULL)
 
+DISTCHECK_CONFIGURE_FLAGS = \
+	--with-systemdsystemunitdir=$$dc_install_base/$(systemdsystemunitdir)
+
 pkgconfigdir = $(libdir)/pkgconfig
 pkgconfig_DATA = libosmo-gsup-client.pc