build: Rename have_libsystemd to have_logind

We really only need the logind API which can be provided either by
libsystemd or libelogind.

This also pulls out finding a logind provider from the native backend
only code because we make use of it in the generic backend code as well.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4223>

src/core/window.c

@@ -109,7 +109,7 @@
 #include "wayland/meta-window-xwayland.h"
 #endif
 
-#ifdef HAVE_LIBSYSTEMD
+#ifdef HAVE_LOGIND
 #include <systemd/sd-login.h>
 #endif
 
@@ -6813,7 +6813,7 @@ meta_window_get_pid (MetaWindow *window)
 GFile *
 meta_window_get_unit_cgroup (MetaWindow *window)
 {
-#ifdef HAVE_LIBSYSTEMD
+#ifdef HAVE_LOGIND
   g_autofree char *contents = NULL;
   g_autofree char *complete_path = NULL;
   g_autofree char *unit_name = NULL;

src/meson.build

@@ -135,9 +135,9 @@ if have_wayland
   ]
 endif
 
-if have_libsystemd
+if have_logind
   mutter_pkg_private_deps += [
-    libsystemd_dep,
+    logind_dep,
   ]
 endif
 
@@ -145,7 +145,6 @@ if have_native_backend
   mutter_pkg_private_deps += [
     libinput_dep,
     libgbm_dep,
-    logind_provider_dep,
     xkbcommon_dep,
   ]
 endif