meson: Move libdl_dep to the top level

Instead of letting each directory to find its way to link with libdl,
it is easier to put the check in the top level, so its result can be
used by all directories.

It is a follow-up of https://gitlab.gnome.org/GNOME/glib/merge_requests/810.

gio/tests/meson.build

@@ -140,17 +140,13 @@ if host_machine.system() != 'windows'
   }
 
   if have_rtld_next
-    # FIXME: This list will probably grow; see
-    # https://gitlab.gnome.org/GNOME/glib/issues/1739
-    no_libdl_systems = ['freebsd', 'netbsd', 'openbsd']
-
     gio_tests += {
       'gsocketclient-slow' : {
         'depends' : [
           shared_library('slow-connect-preload',
             'slow-connect-preload.c',
             name_prefix : '',
-            dependencies: cc.find_library('dl', required: not no_libdl_systems.contains(host_machine.system())),
+            dependencies: libdl_dep,
             install_dir : installed_tests_execdir,
             install: installed_tests_enabled,
           )