Make std::jthread support conditional based on C++ 201911L

[sbooth]

This is a draft PR to address uncertain availability of std::jthread in Apple clang.

[farfromrefug]

@sbooth would love this ;)

[farfromrefug]

@sbooth so you didnt go that route? how are you going to fix the build?

[sbooth]

I don't think that there is anything wrong with the build that needs to be fixed in main. It works correctly with Xcode 26 which I consider the baseline configuration.

[farfromrefug]

@sbooth but it wont build on intel mac using xcode 16. It is just one version below :( Forcing people to use xcode 26 is pretty tough. Not even sure i can install xcode 26 on intel

[sbooth]

I'm sure whether Xcode 26 is a tough requirement could be debated either way, but Xcode 26 does work on Intel Macs. I have an older Intel model and I'm using Xcode 26 with no issues. The only restriction I know of is that the visionOS SDK requires an ARM processor.

[farfromrefug]

@sbooth still pretty sad to see you force people to use xcode 26. Never seen a library do that kind of "requirement".
Will have to fork and manage it in my fork.
Thanks for the work you ve done

[sbooth]

I'll reopen this PR for further consideration.