Intel compilers define `__cpp_constexpr` to `200704` even in C++14 mode.
This indicates that the `cxx_relaxed_constexpr` feature is not
available.  However, Intel 17 and above document support for it.  In
commit v3.8.0~9^2 (Features: Update features for Intel C++ 17.0.2 on
UNIX, 2017-03-31) we added a special check for this case.  Intel 17 and
19 work.  However, Intel 18 does not seem to work and fails our test
case.  Add a special case to disable the feature for Intel 18.