Update C++14 supporting status for clang.

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

  Works similer to BOOST_CONSTEXPR macro, but expands to constexpr only
  if c++14 relaxed constexpr is available.

Signed-off-by: Kohei Takahashi <flast@flast.jp>
Add tests for BOOST_NO_CXX14 macros.

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