[v3] improve exception text when threads not enabled

[Jonathan Wakely]

On 13 August 2012 18:49, Jonathan Wakely wrote:
> I suppose EOPNOTSUPP might be better, if it's supported everywhere.
> EPERM has the advantage of being a documented error for
> pthread_create.

We do define std::errc::operation_not_supported unconditionally on
most platforms, but not mingw or djgpp.  They don't necessarily
support std::errc::operation_not_permitted either, and so far noone
has made std::thread work on those targets anyway, so that's not a
showstopper.

e.g.

terminate called after throwing an instance of 'std::system_error'
  what():  Enable multithreading to use std::thread: Operation not supported
Aborted