[patch] Let libstdc++ know that VxWorks has_nanosleep

[Olivier Hainque]

Hi Jonathan,

> On 19 Dec 2019, at 12:13, Jonathan Wakely <jwakely@redhat.com> wrote:
> 
> 
> Is there a way to detect that more reliably? Should we replicate the
> test used later in the file, to detect whether the timers are really
> enabled for VxWorks?
> 
>      AC_MSG_CHECKING([for nanosleep])
...
> You're the port maintainer though, so if you think that's not needed
> then the patch is OK for trunk.

Thanks for your feedback Jonathan,

Unfortunately, the AC kind of check is actually less reliable
on VxWorks, in particular for the so called "kernel" mode where
linking a "module" actually performs only a partial link to be
downloaded on a target where the run-time loader takes care of
the rest.

The current assumption on the environment configuration is on
a component included by default and required for a number
of very common time related operations anyway, so always there
in practice in our experience.

The rare/unlikely users that would not have the library
configured in on the VxWorks end would just not access to the
c++ time related facilities, which is not a problem.

Overall, this really seems like a good setting for
--enable-libstdcxx-time auto.

Olivier