Get wayland-session wrapper from environment.

diff --git a/daemon/gdm-session.c b/daemon/gdm-session.c
index 3b83fcd3..e60e6a57 100644
--- a/daemon/gdm-session.c
+++ b/daemon/gdm-session.c
@@ -2933,8 +2933,9 @@ gdm_session_start_session (GdmSession *self,
                                                            allow_remote_connections? "--allow-remote-connections " : "",
                                                            command);
                         } else {
-                                program = g_strdup_printf (LIBEXECDIR "/gdm-wayland-session %s\"%s\"",
+                                program = g_strdup_printf (LIBEXECDIR "/gdm-wayland-session %s\"%s %s\"",
                                                            register_session ? "--register-session " : "",
+                                                           g_getenv ("GDM_WAYLAND_SESSION"),
                                                            command);
                         }
                 } else if (run_xsession_script) {
@@ -2965,8 +2966,9 @@ gdm_session_start_session (GdmSession *self,
                                                            register_session ? "--register-session " : "",
                                                            self->selected_program);
                         } else {
-                                program = g_strdup_printf (LIBEXECDIR "/gdm-wayland-session %s\"dbus-run-session -- %s\"",
+                                program = g_strdup_printf (LIBEXECDIR "/gdm-wayland-session %s\"dbus-run-session -- %s %s\"",
                                                            register_session ? "--register-session " : "",
+                                                           g_getenv ("GDM_WAYLAND_SESSION"),
                                                            self->selected_program);
                         }
                 } else {
b662ce33ea419c01c27cf862d8355315912c231'>root</a>/<a href='/guix/log/ROADMAP?id=6b662ce33ea419c01c27cf862d8355315912c231'>ROADMAP</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/ROADMAP?id=6b662ce33ea419c01c27cf862d8355315912c231&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2016-01-29 20:21:54 +0300'>2016-01-29</span></td><td><a href='/guix/commit/ROADMAP?id=171a0a13a4350ac9bbed0f2ed8e9c9777cc57b22'>Rename 'dmd' to 'shepherd' in comments and strings.</a><span class='msg-avail'>...</span></td><td>Alex Kost</td></tr>