The `__cplusplus` macro is not defined to the language level reliably.
When simulating MSVC it is always set to `1`.  When simulating GCC it is
set to the lower value of the current language level and the maximum
level supported by the simulated version of GCC (from observation).

For MSVC we already used a combination of `__INTEL_CXX11_MODE__` and
`__cpp_aggregate_nsdmi` to detect C++14 mode.  Extend this for general
use by removing the condition on `_MSC_VER`.