[mutter] build: Make libsystemd an optional dependency

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit 2ac7f7f1e5bf6381430d0224e7ee51c701664a69
Author: Tom Briden <tom decompile me uk>
Date:   Mon Mar 11 19:49:41 2019 +0000

    build: Make libsystemd an optional dependency
    
    Otherwise it errors out before checking for elogind instead
    
    https://gitlab.gnome.org/GNOME/mutter/merge_requests/491

 meson.build | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/meson.build b/meson.build
index dc210e6fe..6ce9e59f2 100644
--- a/meson.build
+++ b/meson.build
@@ -178,7 +178,7 @@ if have_native_backend
   libgbm_dep = dependency('gbm', version: gbm_req)
   libinput_dep = dependency('libinput', version: libinput_req)
 
-  libsystemd_dep = dependency('libsystemd')
+  libsystemd_dep = dependency('libsystemd', required: false)
   if libsystemd_dep.found()
     logind_provider_dep = libsystemd_dep
   else