[boost] [config] Changes to reflect Visual C++ "15" preview 5

Subject: [boost] [config] Changes to reflect Visual C++ "15" preview 5
From: Beman Dawes (bdawes_at_[hidden])
Date: 2016-10-07 17:38:56

Boost.Config is current reporting these macros for msvc 14.0 update 3 and
later:

BOOST_NO_COMPLETE_VALUE_INITIALIZATION

Xiang Fan of the VC++ compiler team kindly tested "15" preview 5 against
boost without this macro is still getting errors. So this one is still
needed.

BOOST_NO_CXX11_HDR_CODECVT

That's wrong. <codecvt> has been supplied at least since msvc 10.0. I'll
submit a P/R.

BOOST_NO_CXX14_AGGREGATE_NSDMI

Here is a test program, based on the config docs:

#include <cassert>

struct Foo
{
  int x, y = 42;
};

int main()
{
Foo foo = { 1234 };
assert(foo.y == 42);
assert(foo.x == 1234);
}

That now passed on "15" preview 5, so I'll summit a P/R.

BOOST_NO_CXX14_CONSTEXPR

The "15" preview 5 release notes say "The most notable compiler changes
include complete support for generalized constexpr"  I know from
conversations with msft folks that they put a lot of effort into constexpr
fixes, and that they would like bug reports if anyone is finds constexpr
bugs with "15" preview 5. I'll submit a P/R.

BOOST_NO_TWO_PHASE_NAME_LOOKUP

This one still applies. The backstory has changed a lot, however. Enough
progress has been made on the new parser that  two phase name lookup should
ship next year, and maybe even early next year. But don't hold your breath
quite yet.

Thanks to Andrew Pardoe for fact checking a draft of this post.

--Beman