[gtk] Print a warning if GTK_DEBUG is set but gtk isn't built with G_ENABLE_DEBUG

[Christoph Reiter <creiter src gnome org>]

commit 94d9088034e0ad4af69d4baa2fa71127410be078
Author: Christoph Reiter <reiter christoph gmail com>
Date:   Wed Oct 2 21:35:20 2019 +0200

    Print a warning if GTK_DEBUG is set but gtk isn't built with G_ENABLE_DEBUG
    
    G_ENABLE_DEBUG is tied to the meson builttype property, so building with "plain"
    results in G_ENABLE_DEBUG not being defined and the GTK_DEBUG env var just gets ignored
    for that build.
    
    Since it can be confusing that GTK_DEBUG has no effect print a warning message instead.
    
    See #2020. This is a port of !1109 to master

 gtk/gtkmain.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/gtk/gtkmain.c b/gtk/gtkmain.c
index 2d1ab71af5..7d3f43bc9b 100644
--- a/gtk/gtkmain.c
+++ b/gtk/gtkmain.c
@@ -605,17 +605,19 @@ do_pre_parse_initialization (void)
 
   gdk_pre_parse ();
 
-#ifdef G_ENABLE_DEBUG
   env_string = g_getenv ("GTK_DEBUG");
   if (env_string != NULL)
     {
+#ifdef G_ENABLE_DEBUG
       debug_flags[0].flags = g_parse_debug_string (env_string,
                                                    gtk_debug_keys,
                                                    G_N_ELEMENTS (gtk_debug_keys));
       any_display_debug_flags_set = debug_flags[0].flags > 0;
+#else
+      g_warning ("GTK_DEBUG set but ignored because gtk isn't built with G_ENABLE_DEBUG");
+#endif  /* G_ENABLE_DEBUG */
       env_string = NULL;
     }
-#endif  /* G_ENABLE_DEBUG */
 
   env_string = g_getenv ("GTK_SLOWDOWN");
   if (env_string)