Define BOOST_NO_CXX14_CONSTEXPR unless clang > 3.4.

Signed-off-by: Kohei Takahashi <flast@flast.jp>
Merge pull request #42 from Flast/bugfix/cxx14/clang34-constexpr

Define BOOST_NO_CXX14_CONSTEXPR unless clang > 3.4.