call_with_sigsetjmp should be toggleable via a feature flag

[pnkfelix]

I hadn't mean for call_with_sigsetjmp to be part of the default API for this crate. It just was something that seemed like some users would need and that would be "easy" to add (but maybe not as easy as you might think...)

[jeff-davis]

My use case needs call_with_sigsetjmp. Why is it important to have a feature flag here? I don't mind, but I also don't see the cost of leaving it there by default.

[pnkfelix]

I don't mind leaving it in as part of the default feature set.

I just imagine there may be some niche targets that have setjmp but not sigsetjmp; it seems trivial to make a flag to toggle the build to leave out sigsetjmp for that case.