Add support for -fno-exceptions

[void2012]

I want to thank you @tcbrindle for this amazing library! I find it way easier to use than std::ranges. However, I have a platform that has no exceptions support. And want to reuse my flux code here, but it doesn't compile because of exceptions.

There are only two places where C++ exception handling is used:

auto flux::generator<ElemT>::promise_type::unhandled_exception() 
{ throw; } 

void flux::detail::runtime_error_fn::unwind(const char* msg, std::source_location loc)
{
        char buf[1024];
        std::snprintf(buf, 1024, "%s:%u: Fatal error: %s",
                      loc.file_name(), loc.line(), msg);
        throw unrecoverable_error(buf);
}

As there are no try-catch clauses in the library, I think it's safe to use either std::terminate or __builtin_trap as a -fno-exceptions alternative. Please add a check for -fno-exceptions with something like __cpp_exceptions or __EXCEPTIONS
Thanks!

[void2012]

It also would be great to have a custom termination handler

[tcbrindle]

Hi @void2012,

Supporting an -fno-exceptions mode seems like a reasonable thing to do.

I don't think any change should be needed for generator (there shouldn't be any unhandled exceptions if exceptions are disabled!) but I could be wrong.

The second function is easy enough to #define away if the library is configured not to use exceptions. Obviously unwinding won't be available in this case, but you can still use FLUX_TERMINATE_ON_ERROR (the default) or FLUX_FAIL_FAST_ON_ERROR as today.

It also would be great to have a custom termination handler

I've considered allowing some sort of custom error handler, but I'm not sure yet what the best approach would be if we want to support modules as well as headers.