Adds thread-safety for trees that are accessed on only one thread

[PTNobel]

Best I can tell from going through the code, the only state that is shared across trees is the alloc variables; so this makes them thread locals.

To access thread locals, I bump the C version to C11. Every platform CVXPY supports supports C11, so I figured this would be not an issue. LMK if I am worng.

[PTNobel]

This is I think the only PR needed on the base engine to make the python library support free-threading.

[PTNobel]

Honestly, I think we should move to C17 or C23. I don't think we gain anything from the old standards

[dance858]

Can you explain what you are trying to do?

[PTNobel]

I'm trying to add free threading support to CVXPY. This means we need to have a thread safety story for all our dependencies. Looking through this library, it seemed to have only one dependency on shared state, so I was trying to eliminate that so we could have a clear thread safety story here.

I will then add free-threading support in the Python bindings.

[PTNobel]

I'm a little confused what is wrong in CI... It worked locally Fixed. I forgot to push one file.

[PTNobel]

I think my expectation at this point is "if a expression tree is only ever accessed from one thread, this library is thread-safe" would be the story I'm after here. We'll have to do some locking at the python level; but I think this would be a good start to get rid of the obvious issues.