bumblebee: add systemd build option (off)

srcpkgs/bumblebee/template

@@ -1,7 +1,7 @@
 # Template file for 'bumblebee'
 pkgname=bumblebee
 version=3.2.1
-revision=3
+revision=4
 build_style=gnu-configure
 configure_args="
 	CONF_DRIVER_MODULE_NVIDIA=nvidia
@@ -13,7 +13,6 @@ configure_args="
 hostmakedepends="pkg-config help2man"
 makedepends="VirtualGL-devel glib-devel MesaLib-devel"
 depends="VirtualGL"
-systemd_services="bumblebeed.service true"
 conf_files="
 	/etc/bumblebee/bumblebee.conf
 	/etc/bumblebee/xorg.conf.nouveau
@@ -28,10 +27,16 @@ checksum=1018703b07e2f607a4641249d69478ce076ae5a1e9dd6cff5694d394fa7ee30e
 
 only_for_archs="i686 x86_64"
 
+build_options="systemd"
+if [ "$build_option_systemd" ]; then
+	systemd_services="bumblebeed.service true"
+fi
+
 post_install() {
-	# Install systemd unit
-	vinstall scripts/systemd/bumblebeed.service 644 \
-		usr/lib/systemd/system
+	# Install systemd service
+	if [ "$build_option_systemd" ]; then
+		vinstall scripts/systemd/bumblebeed.service 644 usr/lib/systemd/system
+	fi
 
 	# Make bash_completion work
 	vmkdir usr/share/bash-completion/completions