Avoid the warning that tmpname should be replaced by mkstemp by
replacing tmpname with mkstemp. lua has code to do that but it isn't
trivial to activate while leaving the lua distribution unmodified.
[I've no idea whether nethack's usage even results in liblua ever
creating any temporary files.]
Using nethack's hints to pass -DLUA_USE_POSIX from top Makefile to
lib/lua-$(LUA_VERSION)/Makefile has been tested, building liblua.a
on linux with that has not.
ce5eebcb9b