rand is not ISO C compliant in Cygwin

[Bruno Haible]

Norton Allen wrote:
> Cygwin/(newlib?) has chosen to avoid race conditions by making 
> pseudo-random sequences in different threads independent. Although the 
> standard does not require this, it does not prohibit it either.

I disagree. I cited the relevant sentences from the standard.

Other platforms (glibc, Android) avoid race conditions in rand()
by locking. Which is compliant with the standard.

Bruno