Ken Brown wrote:
>> I installed the attached patch into the Emacs master, to try to work
>> around this problem by putting GCC into C11 mode. Please give it a try.
>
> No, that didn't fix it.

Can you investigate why not?  The patch should cause 'configure' to put 
-stdc=gnu11 into CFLAGS. If you look at config.log, it should have something 
like this:

configure:5924: checking for gcc option to enable C11 features
...
configure:6146: result: -std=gnu11

If this isn't the result, what is the result and why?

> What about an ad hoc temporary solution that simply defines __ISO_C_VISIBLE to be 2011 on Cygwin prior to the inclusion of <stdlib.h>?

I'd rather not go that route, as __ISO_C_VISIBLE is supposed to be private. I 
installed the attached patch instead; please give it a try.