Add compile time warnings when using an unsupported compiler

As a user of libcu++, I'd like to know if I'm using an unsupported compiler.

A compile time warning/error would be preferred as it maximizes the likelihood that I will see I'm using an unsupported environment.

However, there should also be a way to disable the warning/error if I am willing to accept the risk of using an unsupported compiler.

I propose we modify the `__config` header with something like the following:
```
#if !defined(LIBCUDACXX_DISABLE_UNSUPPORTED_COMPILER_WARNING) && /* ( gcc < min_gcc || clang < min_clang || msvc < min_msvc) */
#warning "Detected an unsupported compiler version. Use at your own risk. This warning can be silenced by defining `LIBCUDACXX_DISABLE_UNSUPPORTED_COMPILER_WARNING."
#endif
```