Windows -- including Cygwin -- return a NaN representation with the sign bit set, whereas other platforms return one without the sign bit set. This results in different binary output in the JSON-to-binary part of capnp-test.sh.

This was uncovered since we're now runing capnp-test.sh for Cygwin builds, but the problem actually applies to *all* Windows builds. We just weren't running this test for the other builds before.

Interestingly, I *had* run this test with a MinGW cross-compile build running on WINE, and it passed there. Seemingly, WINE's strtod() directly wraps the host system's without trying to emulate Windows differences.