D7: editor multi-cursor support

Add secondary_cursors field to Editor with insert_char_multi,
delete_back_multi, delete_forward_multi methods. Right-to-left
processing ensures position shifts don't corrupt earlier insertions.

7 new tests: add/clear, all_positions, insert, delete_back,
delete_forward, unicode, duplicate-add.

local/recipes/tools/diffutils/source/m4/tm_gmtoff.m4

@@ -0,0 +1,14 @@
+# tm_gmtoff.m4 serial 3
+dnl Copyright (C) 2002, 2009-2017 Free Software Foundation, Inc.
+dnl This file is free software; the Free Software Foundation
+dnl gives unlimited permission to copy and/or distribute it,
+dnl with or without modifications, as long as this notice is preserved.
+
+AC_DEFUN([gl_TM_GMTOFF],
+[
+ AC_CHECK_MEMBER([struct tm.tm_gmtoff],
+                 [AC_DEFINE([HAVE_TM_GMTOFF], [1],
+                            [Define if struct tm has the tm_gmtoff member.])],
+                 ,
+                 [#include <time.h>])
+])