add login-proxy option

There are cases where systemd-logind is not used/running. Result is
that bcklight module will not run.

Add an option that, when set to false, allows backlight module to
work without systemd-logind.

meson.build

@@ -333,6 +333,10 @@ if get_option('niri')
     )
 endif
 
+if get_option('login-proxy')
+    add_project_arguments('-DHAVE_LOGIN_PROXY', language: 'cpp')
+endif
+
 if libnl.found() and libnlgen.found()
     add_project_arguments('-DHAVE_LIBNL', language: 'cpp')
     src_files += files('src/modules/network.cpp')