maint: preliminary Lua 5.3.0-beta compatibility.

Close #181.
* configure.ac (AX_PROG_LUA): Accept Lua 5.3 interpreters.
* ext/include/lua52compat.h: Remove.
* local.mk (EXTRA_DIST): Adjust accordingly.
* ext/include/_helpers.c (lua_objlen, lua_strlen, luaL_openlib)
(luaL_register): Lua 5.2 and 5.3 compatibility definitions.
* ext/posix/sched.c, ext/posix/termio.c: Use luaposix optint()
rather than deprecated luaL_optint().
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>