Fix -Wundef warnings when building liblua

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).

Reviewed By: kevans
Differential Revision: https://reviews.freebsd.org/D25977