[v3 PATCH] Implement std::optional.

[Jonathan Wakely]

On 07/07/16 13:00 +0300, Ville Voutilainen wrote:
>    Implement std::optional.
>    * include/Makefile.am: Add optional to exported headers.
>    * include/Makefile.in: Likewise.
>    * include/std/optional: New.
>

All the references to [X.Y.4] could refer to something sensible now,
as we know where it landed in the CD. Using stable names might be
better than instable clause numbers though. Every time I find a 20.a.b
refernce in our headers it's wrong :-(  I've either been changing them
to "C++11 20.c.d" so they are anchored to a particular standard, or
using stable names. We should change those comments in
<experimental/optional> some time too.

There are also a small number of redundant std::-qualifications on
true_type, false_type, remove_cv_t and remove_reference_t, but no
harm.

I'm also not sure why the testsuite/experimental/optional/* tests have
{ dg-do run} which is also redundant, but not harm again.

OK for trunk.