Update for MSVC-14 CTP14.
Fix <type_traits> tests to check for the correct names!
Enable C++14 return type deduction for vc14.
Presence of pthread_yield is dependent on __USE_GNU.
Define BOOST_NO_CXX14_CONSTEXPR unless clang > 3.4.

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