Commit 957016621e ("Remove support for
Boost.TypeTraits") removed support for any alternative implementation of
<type_traits>, so C++11 <type_traits> is now required. Move it to
the Cxx11RequiredFeature array to combine it with the other required
feature tests.
Boost enable_if works differently from std::enable_if, so the build is
broken when using Boost.TypeTraits. The broken code has been present
for a long time and no one reported it. Remove the fallback to Boost.
