add codeset.m4, OS X doesn't have this

m4/codeset.m4

+# codeset.m4 serial AM1 (gettext-0.10.40)
+dnl Copyright (C) 2000-2002 Free Software Foundation, Inc.
+dnl This file is free software, distributed under the terms of the GNU
+dnl General Public License.  As a special exception to the GNU General
+dnl Public License, this file may be distributed as part of a program
+dnl that contains a configuration script generated by Autoconf, under
+dnl the same distribution terms as the rest of that program.
+
+dnl From Bruno Haible.
+
+AC_DEFUN([AM_LANGINFO_CODESET],
+[
+  AC_CACHE_CHECK([for nl_langinfo and CODESET], am_cv_langinfo_codeset,
+    [AC_TRY_LINK([#include <langinfo.h>],
+      [char* cs = nl_langinfo(CODESET);],
+      am_cv_langinfo_codeset=yes,
+      am_cv_langinfo_codeset=no)
+    ])
+  if test $am_cv_langinfo_codeset = yes; then
+    AC_DEFINE(HAVE_LANGINFO_CODESET, 1,
+      [Define if you have <langinfo.h> and nl_langinfo(CODESET).])
+  fi
+])