fix building with -Dsystemd=auto and no systemd

`-Dsystemd=auto` previously worked only if systemd was present, i.e.
synonymous to `-Dsystemd=enabled`. otherwise, the build would error:

```
> Run-time dependency systemd found: NO (tried pkgconfig)
>
> meson.build:111:12: ERROR: Dependency "systemd" not found, tried pkgconfig
```

meson.build

@@ -108,12 +108,14 @@ if build_modules
 endif
 
 if build_modules
-  systemd = dependency('systemd', required: get_option('systemd'))
-  libsystemd_dep = dependency('libsystemd',required: get_option('systemd'))
-  libelogind_dep = dependency('libelogind', required: get_option('elogind'))
-  summary({'systemd conf data': systemd.found(),
-          'libsystemd': libsystemd_dep.found(),
-          'libelogind': libelogind_dep.found()}, bool_yn: true)
+  summary({'systemd conf data required?': get_option('systemd'),
+          'elogind': get_option('elogind')})
+  systemd = dependency('systemd', required: false)
+  libsystemd_dep = dependency('libsystemd', required: false)
+  libelogind_dep = dependency('libelogind', required: false)
+  # summary({'systemd conf data': systemd.found(),
+  #         'libsystemd': libsystemd_dep.found(),
+  #         'libelogind': libelogind_dep.found()}, bool_yn: true)
 endif
 
 gnome = import('gnome')