tig: Don't override XML_CATALOG_FILES

tig/patches/patch-contrib_config.make-Darwin

@@ -0,0 +1,16 @@
+$NetBSD: patch-contrib_config.make-Darwin,v 1.1 2014/05/15 01:14:44 pho Exp $
+
+Do not override XML_CATALOG_FILES on Darwin. That will only make
+xmllint try to find a wrong catalog.
+
+--- contrib/config.make-Darwin.orig	2018-01-31 01:13:24.000000000 +0000
++++ contrib/config.make-Darwin
+@@ -1,8 +1,5 @@
+ # Example configuration for Darwin / Mac OS X.
+ 
+-# Work-around for Homebrew-based xmlto.
+-export XML_CATALOG_FILES=/usr/local/etc/xml/catalog
+-
+ TIG_CFLAGS += -DHAVE_EXECINFO_H
+ TIG_LDLIBS = -liconv
+