Some compilers try very had to pretend they're another compiler (ICC
pretends to be GCC and Linux and MacOS, and MSVC on windows), Clang
behaves much like GCC, but now also has clang-cl, which behaves like MSVC.
This method provides an easy way to determine whether testing for MSVC
like arguments `/w1234` or gcc like arguments `-Wfoo` are likely to
succeed, without having to check for dozens of compilers and the host
operating system, (as you would otherwise have to do with ICC).
This allows us to more easily have the documentation in sync with
the source code as people will have to document new features etc
right at the time where they implement it.