Makefile: build with systemd when available

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

Makefile

@@ -3,7 +3,7 @@ export GOPROXY=https://proxy.golang.org
 APPARMORTAG := $(shell hack/apparmor_tag.sh)
 STORAGETAGS := $(shell ./btrfs_tag.sh) $(shell ./btrfs_installed_tag.sh) $(shell ./libdm_tag.sh) $(shell ./hack/libsubid_tag.sh)
 SECURITYTAGS ?= seccomp $(APPARMORTAG)
-TAGS ?= $(SECURITYTAGS) $(STORAGETAGS)
+TAGS ?= $(SECURITYTAGS) $(STORAGETAGS) $(shell ./hack/systemd_tag.sh)
 BUILDTAGS += $(TAGS)
 PREFIX := /usr/local
 BINDIR := $(PREFIX)/bin

hack/systemd_tag.sh

@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+${CPP:-${CC:-cc} -E} ${CPPFLAGS} - > /dev/null 2> /dev/null << EOF
+#include <systemd/sd-daemon.h>
+EOF
+if test $? -eq 0 ; then
+	echo systemd
+fi