new trunk dependence on nanosleep()

[Tim Prince]

I see that the current trunk chooses to configure so as to depend on a
compatible nanosleep() regardless of the ability of the target system to
support that (e.g. cygwin64).  I haven't been able to see whether there
is a configure option or a prerequisites download installation to take
care of it.
The reported compile error has to do with undefined errno and associated
macros in the nanosleep() call.  Is there an include missing from the
libstdc++ source where it calls nanosleep() ?