Install systemd services with autotools

Change-Id: I563559f5b501eded44efafc60bb0c9ffdea20b3e
diff --git a/Makefile.am b/Makefile.am
index f9849c0..be378be 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -1,5 +1,5 @@
 ## Process this file with automake to produce Makefile.in
-SUBDIRS = lib gtp ggsn sgsnemu doc tests
+SUBDIRS = lib gtp ggsn sgsnemu doc contrib tests
 
 pkgconfigdir = $(libdir)/pkgconfig
 pkgconfig_DATA = libgtp.pc
@@ -12,4 +12,7 @@
 
 EXTRA_DIST = git-version-gen .version README.md README.FreeBSD README.MacOSX
 
+DISTCHECK_CONFIGURE_FLAGS = \
+	--with-systemdsystemunitdir=$$dc_install_base/$(systemdsystemunitdir)
+
 @RELMAKE@