replace nested functions with blocks on clang

[cute]

No description provided.

[orangeduck]

Thanks this looks great 👍 Can you put it behind another processor flag as well as __clang__? Some people may either be using clang already with nested functions or not want this behaviour.

[XVilka]

Are there any updates on this PR?

[orangeduck]

Are you interested in using clang blocks for your own project? I can merge this in if there is some interest - just need to add an extra pre-processor flag to guard it.

[sjml]

FWIW, clang doesn't support GCC-style nested functions at all, so there wouldn't be anybody using clang with LuaAutoC from what I can tell. Thus there wouldn't be a need for an additional guard. Using blocks might be the only way to get it building on clang.

[orangeduck]

You can use clang with LuaAutoC you just need to have separate declaration and registration functions as mentioned in the FAQ under "nested functions": https://github.com/orangeduck/LuaAutoC#faq but yes in general in clang you need to use blocks instead of nested functions.

[sjml]

Ah, yes, my bad. Sorry I spoke overly-definitively!