chore(linux): improve compatibility with Gentoo Linux

Gentoo doesn't use systemd. `libelogind` implements some aspects of
systemd, so we use that on Gentoo (or rather when `libsystemd` is not
available but `libelogind` is).

Patch from https://forums.gentoo.org/viewtopic.php?p=8850566.
See also https://community.software.sil.org/t/keyman-for-gentoo-linux/9615.

linux/ibus-keyman/meson.build

@@ -8,7 +8,7 @@ conf = configuration_data()
 ibus =           dependency('ibus-1.0',             version: '>= 1.2.0')
 gtk =            dependency('gtk+-3.0',             version: '>= 2.4')
 json_glib =      dependency('json-glib-1.0',        version: '>= 1.0')
-systemd  =       dependency('libsystemd')
+systemd  =       dependency('libsystemd', 'libelogind')
 icu =            dependency('icu-i18n')
 
 core_dir = meson.current_source_dir() / '../../core'

linux/keyman-system-service/meson.build

@@ -4,7 +4,7 @@ project('keyman-system-service', 'c', 'cpp',
         meson_version: '>=0.61')
 
 evdev    = dependency('libevdev', version: '>= 1.9')
-systemd  = dependency('libsystemd')
+systemd  = dependency('libsystemd', 'libelogind')
 
 subdir('resources')
 subdir('src')

linux/keyman-system-service/resources/com.keyman.SystemService1.service.libelogind

@@ -0,0 +1,7 @@
+# /usr/share/dbus-1/system-services/com.keyman.SystemService1.service
+# This version used with elogind (e.g. on Gentoo)
+
+[D-BUS Service]
+Name=com.keyman.SystemService1
+Exec=/usr/libexec/systemd-keyman.service
+User=root

linux/keyman-system-service/resources/com.keyman.SystemService1.service → linux/keyman-system-service/resources/com.keyman.SystemService1.service.libsystemd

@@ -1,4 +1,5 @@
 # /usr/share/dbus-1/system-services/com.keyman.SystemService1.service
+# This version used with systemd (e.g. on Debian)
 
 [D-BUS Service]
 Name=com.keyman.SystemService1

linux/keyman-system-service/resources/meson.build

@@ -1,3 +1,3 @@
 install_data('com.keyman.SystemService1.conf', install_dir: get_option('datadir') / 'dbus-1/system.d/')
-install_data('com.keyman.SystemService1.service', install_dir: get_option('datadir') / 'dbus-1/system-services/')
+install_data('com.keyman.SystemService1.service.' + systemd.name(), install_dir: get_option('datadir') / 'dbus-1/system-services/', rename: ['com.keyman.SystemService1.service'])
 install_data('systemd-keyman.service', install_dir: get_option('prefix') / 'lib/systemd/system/')