Add ability to use a lua library compiled as C++

[LiquidFenrir]

This is for my (admittedly specific) use case, but also an option that might be good to have.
It is rarely the case, as all the system packaged lua libraries I know of are compiled as C, but for an in-project compiled lua it can happen; since it allows for exceptions to work properly through the lua interface.

In the current master, the extern "C" would result in linking errors (undefined references for every single lua function).
Set to OFF by default to keep compatibility.

I'm using RmlUi alongside sol2, which combines nicely, but I prefer having all the safeties enabled - including exceptions. This is the only thing (alongside a dummy FindLua.cmake for some reason, otherwise RmlUi uses the system library) needed to make it work.

Let me know if this is a bad change or if there are naming conventions you'd want fixed.
I also noticed the cmake branch, don't really know where to PR this. But it's not hard to port over, so master it is.

[mikke89]

Ah, I didn't know Lua could be compiled as C++ to give exceptions. I see that this is officially supported and they even mention the conditional use of the "C" wrapper.

First time I'm hearing about the need for this, but since it's officially supported from the Lua side, it makes sense to add it as an option. So thank you for the pull request, and this looks good to me!

I'll have to remember to add this to the cmake branch as well.