[gtk/cups-build-fix] Fix the build with cups < 2.3

[Matthias Clasen <matthiasc src gnome org>]

commit d761e3cf2c31b91269a3dbfd64a553decdeec51c
Author: Matthias Clasen <mclasen redhat com>
Date:   Fri Jul 31 10:00:20 2020 -0400

    Fix the build with cups < 2.3
    
    PPD_CUSTOM_UNKNOWN was introduced in cups 2.3.
    Only handle it when we build against a new enough cups.
    
    Fixes: #2968

 modules/printbackends/gtkprintbackendcups.c | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/modules/printbackends/gtkprintbackendcups.c b/modules/printbackends/gtkprintbackendcups.c
index 6453e824d9..f3435b57bb 100644
--- a/modules/printbackends/gtkprintbackendcups.c
+++ b/modules/printbackends/gtkprintbackendcups.c
@@ -647,7 +647,9 @@ add_cups_options (const char *key,
                           custom_value = TRUE;
                           break;
 
+#if CUPS_VERSION_MAJOR == 2 && CUPS_VERSION_MINOR >= 3
                         case PPD_CUSTOM_UNKNOWN:
+#endif
                         default :
                           custom_value = FALSE;
                         }