cups: Include TargetConditionals.h on Darwin

pkgs/misc/cups/0001-TargetConditionals.patch

···
+From 1204c841999808ba27267a0039777dcbccdcd6e3 Mon Sep 17 00:00:00 2001
+From: toonn <[e-mail]>
+Date: Sun, 27 Jun 2021 12:30:08 +0200
+Subject: [PATCH] TargetConditionals
+
+---
+ test/ippserver.c | 6 +++++-
+ 1 file changed, 5 insertions(+), 1 deletion(-)
+
+diff --git a/test/ippserver.c b/test/ippserver.c
+index 38b304f..68ccab1 100644
+--- a/test/ippserver.c
++++ b/test/ippserver.c
+@@ -25,7 +25,11 @@
+  * Include necessary headers...
+  */
+ 
+-#include <config.h>			/* CUPS configuration header */
++#ifdef __APPLE__
++#  include <xcode/config.h>		/* CUPS configuration header macOS */
++#else
++#  include <config.h>			/* CUPS configuration header */
++#endif /* __APPLE__ */
+ #include <cups/cups.h>			/* Public API */
+ #include <cups/string-private.h>	/* CUPS string functions */
+ #include <cups/thread-private.h>	/* For multithreading functions */
+-- 
+2.17.2 (Apple Git-113)
+

pkgs/misc/cups/default.nix

···
 
   outputs = [ "out" "lib" "dev" "man" ];
 
+  patches = lib.optional (version == "2.2.6") ./0001-TargetConditionals.patch;
+
   postPatch = ''
     substituteInPlace cups/testfile.c \
       --replace 'cupsFileFind("cat", "/bin' 'cupsFileFind("cat", "${coreutils}/bin'