build: Add --with-systemdsystemunitdir configure option

Makefile.am

@@ -2,6 +2,7 @@ SUBDIRS = data src docs
 
 DISTCHECK_CONFIGURE_FLAGS =								\
 	--enable-gtk-doc								\
-	--with-udevrulesdir=$$dc_install_base/$(udevrulesdir)
+	--with-udevrulesdir=$$dc_install_base/$(udevrulesdir)				\
+	--with-systemdsystemunitdir=$$dc_install_base/$(systemdsystemunitdir)
 
 EXTRA_DIST = README.md

configure.ac

@@ -22,6 +22,14 @@ AC_ARG_WITH([udevrulesdir],
 	    [with_udevrulesdir=$($PKG_CONFIG --variable=udevdir udev)"/rules.d"])
 AC_SUBST([udevrulesdir], [$with_udevrulesdir])
 
+AC_ARG_WITH([systemdsystemunitdir],
+	    AS_HELP_STRING([--with-systemdsystemunitdir=DIR], [Directory for systemd service files]),
+	    [],
+	    [with_systemdsystemunitdir=$($PKG_CONFIG --variable=systemdsystemunitdir systemd)])
+if test x$with_systemdsystemunitdir != xno; then
+	AC_SUBST([systemdsystemunitdir], [$with_systemdsystemunitdir])
+fi
+
 PKG_CHECK_MODULES(IIO_SENSOR_PROXY, gio-2.0 gudev-1.0)
 
 AC_PATH_PROG([GDBUS_CODEGEN],[gdbus-codegen])

data/Makefile.am

@@ -1,12 +1,13 @@
 udevrules_DATA = 40-iio-sensor-proxy.rules
 
-servicedir = /usr/lib/systemd/system/
-service_DATA = iio-sensor-proxy.service
+systemdservicedir = $(systemdsystemunitdir)
+systemdservice_in_files = iio-sensor-proxy.service.in
+systemdservice_DATA = iio-sensor-proxy.service
 iio-sensor-proxy.service: iio-sensor-proxy.service.in Makefile
 	$(AM_V_GEN) $(SED) -e "s|\@sbindir\@|$(sbindir)|" $< > $@
 
 dbus_confdir = /etc/dbus-1/system.d
 dbus_conf_DATA = net.hadess.SensorProxy.conf
 
 CLEANFILES = iio-sensor-proxy.service
-EXTRA_DIST = $(udevrules_DATA) $(dbus_conf_DATA) iio-sensor-proxy.service.in
+EXTRA_DIST = $(udevrules_DATA) $(dbus_conf_DATA) $(systemdservice_in_files)