[gnome-session] Revert "main: Remove GNOME_DESKTOP_SESSION_ID envvar"

[Ray Strode <halfline src gnome org>]

commit 5f920253dfb2c0f2c4b742690f306cfea2ac6613
Author: Ray Strode <rstrode redhat com>
Date:   Mon Feb 12 14:38:07 2018 -0500

    Revert "main: Remove GNOME_DESKTOP_SESSION_ID envvar"
    
    This reverts commit 6bf47ad8c6f0a513c95a5043e38ad51cc0248943.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=790387

 gnome-session/main.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)
---
diff --git a/gnome-session/main.c b/gnome-session/main.c
index 9a7f554..b1ac385 100644
--- a/gnome-session/main.c
+++ b/gnome-session/main.c
@@ -408,6 +408,11 @@ main (int argc, char **argv)
                 g_free (ibus_path);
         }
 
+        /* Some third-party programs rely on GNOME_DESKTOP_SESSION_ID to
+         * detect if GNOME is running. We keep this for compatibility reasons.
+         */
+        gsm_util_setenv ("GNOME_DESKTOP_SESSION_ID", "this-is-deprecated");
+
         /* We want to use the GNOME menus which has the designed categories.
          */
         gsm_util_setenv ("XDG_MENU_PREFIX", "gnome-");