Fix -Wundef warnings when building liblua

[arichardson]

Summary

We need to define the LUA_FLOAT_INT64 macro even if we don't use it (copied
from stand/luaconf.h). While touching luaconf.h.dist also sync it with the
the 5.3.5 release version (matches the one in lib/liblua).

[arichardson]

ping?

[kevans]

It's interesting, the luaconf.h used in lualoader *does* have these two definitions, and somehow the luaconf.h I hacked up for flua ended up with one but not the other?

Odd, but LGTM.