Allow to disable MSVC exception workaround

[mtnpke]

gsl_assert automaticaly uses __fastfail instead of std::terminate if it detects that

• the compiler is MSVC and
• exceptions are disabled (either due to compilation in kernel mode or user mode with explicit disabling).

This is fine for the normal case (which I think is MSVC+Dinkumware), but in my case (MSVC+EASTL in kernel mode), it causes problems. I do want custom code to run on contract violation, which I can do by implementing std::terminate in a compatible way. However, I cannot achieve this since the autodetection overrules calling terminate. If I could define GSL_MSVC_USE_STL_NOEXCEPTION_WORKAROUND to 0 beforehand and thereby skip the autodetection, this would be very helpful.