[build2] build2 g++ -std=c++1y problem

[Boris Kolpackov]

Boris Kolpackov <boris at codesynthesis.com> writes:

> > __POSIX_VISIBLE is ruining my pthreading on cygwin
> 
> If you can come up with a fix [...]

BTW, seeing that this is a shared_mutex issue (which I believe is a bug
in GCC and/or Cygwin), there is a way to disable its use (which we also
have to do for older Mac OS versions). The place to look for this is
libbutl/libbutl/ft/shared_mutex.hxx.

Boris