build: make systemd dependency optional

The systemd dependency is only used to install some systemd service
files. This can easily be made optional.

data/meson.build

@@ -11,6 +11,7 @@ configure_file(
     install_dir: dbus_service_dir,
 )
 
+if get_option('systemd')
     configure_file(
         configuration: configuration_data({
             'libexecdir': fprintd_installdir,
@@ -20,6 +21,7 @@ configure_file(
         install: true,
         install_dir: systemd_unit_dir,
     )
+endif
 
 polkit_policy = 'net.reactivated.fprint.device.policy'
 polkit_policy_target = i18n.merge_file(polkit_policy,

meson.build

@@ -94,13 +94,17 @@ pod2man = find_program('pod2man', required: get_option('man'))
 xsltproc = find_program('xsltproc', required: get_option('gtk_doc'))
 
 # StateDirectory was introduced in systemd 235
-systemd_dep = dependency('systemd', version: '>= 235')
+systemd_dep = dependency('systemd', version: '>= 235', required: false)
 systemd_unit_dir = get_option('systemd_system_unit_dir')
 
-if systemd_unit_dir == ''
+if systemd_unit_dir == '' and systemd_dep.found()
     systemd_unit_dir = systemd_dep.get_pkgconfig_variable('systemdsystemunitdir')
 endif
 
+if get_option('systemd') and systemd_unit_dir == ''
+    error('systemd development files or systemd_system_unit_dir is needed for systemd support.')
+endif
+
 dbus_service_dir = get_option('dbus_service_dir')
 dbus_data_dir = datadir
 dbus_interfaces_dir = ''

meson_options.txt

@@ -6,6 +6,10 @@ option('man',
     description: 'Generate the man files',
     type: 'boolean',
     value: true)
+option('systemd',
+    description: 'Install system service files',
+    type: 'boolean',
+    value: true)
 option('systemd_system_unit_dir',
     description: 'Directory for systemd service files',
     type: 'string')