Define BOOST_NO_CXX14_CONSTEXPR unless clang > 3.4.

clang (< 3.5) has a defet with constexpr and dependent type.

- 3.4 http://melpon.org/wandbox/permlink/xe6bdDS3mPPLhbA3 (fail)
- 3.5 http://melpon.org/wandbox/permlink/weorcI0Mb9u8g98r

This issue will afect fusion::swap constexpr supporting.
- [Test output: marshall-mac - fusion - swap / clang-darwin-11](http://www.boost.org/development/tests/develop/developer/output/marshall-mac-boost-bin-v2-libs-fusion-test-swap-test-clang-darwin-11-debug.html)
 - Above test run under c++11 mode, though c++14 mode is also affected.