[gtk: 1/2] Fix the build with cups < 2.2.12

[Matthias Clasen <matthiasc src gnome org>]

commit 0493e89897cab9d8d50c3c1297c346dc4c384907
Author: Thomas Lange <thomas-lange2 gmx de>
Date:   Sat Oct 24 13:16:40 2020 +0200

    Fix the build with cups < 2.2.12
    
    PPD_CUSTOM_UNKNOWN was introduced in cups 2.3.
    and has been backported to 2.2.12.
    Only handle it when we build against a new enough cups.
    
    Fixes: #2968

 modules/printbackends/gtkprintbackendcups.c | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/modules/printbackends/gtkprintbackendcups.c b/modules/printbackends/gtkprintbackendcups.c
index b88d76011c..d499e11f98 100644
--- a/modules/printbackends/gtkprintbackendcups.c
+++ b/modules/printbackends/gtkprintbackendcups.c
@@ -647,7 +647,11 @@ add_cups_options (const char *key,
                           custom_value = TRUE;
                           break;
 
+#if (CUPS_VERSION_MAJOR >= 3) || \
+    (CUPS_VERSION_MAJOR == 2 && CUPS_VERSION_MINOR >= 3) || \
+    (CUPS_VERSION_MAJOR == 2 && CUPS_VERSION_MINOR == 2 && CUPS_VERSION_PATCH >= 12)
                         case PPD_CUSTOM_UNKNOWN:
+#endif
                         default :
                           custom_value = FALSE;
                         }