Add support for openresty's branch of luajit

[synodriver]

I've created a fork with that support, but have no idea whether to realease a new package or just merge to upstream. Would you consider adding some new apis for that?

[scoder]

I wouldn't oppose it,as long as it doesn't get in the way of me (as a maintainer) and users who do not make use of this.

However, the generated C code still needs to compile with the normal Lua and LuaJIT APIs. That means that, in the "normal" case, you'll have to 1) mark the new functionality as unavailable and 2) find replacement C macros for the API functions.

Note that you don't need cpdef unless you're making module-internal calls to those functions/methods. Normal def functions will do.