First implementations for BOOST_NO_CXX14_x macros.

Signed-off-by: Kohei Takahashi <flast@flast.jp>
Update C++14 supporting status for clang.

Signed-off-by: Kohei Takahashi <flast@flast.jp>
Add BOOST_RELAXED_CONSTEXPR macro.

  Similer to BOOST_CONSTEXPR macro, but make as constexpr only if c++14
  relaxed constexpr is available.

Signed-off-by: Kohei Takahashi <flast@flast.jp>