What does it mean to include pthread.h with -std=c11?

[DJ Delorie]

> The question here is what do _really_ want to do in this case?

IMHO if the user includes one of our headers, we should provide the
requested functionality - somehow - regardless of compiler options.

Or we should hard-fail with a message that says why we aren't.

The BZ seems to indicate that we're somewhere in the middle, where we
act like we're providing a pthreads implementation, but we fail to
provide enough of it to work properly, and give unhelpful errors.  This
makes it *look* like a bug, even if this is intentional.  To me, this is
the worst of the three options, from the user's perspective.