Disable warnings about generated spectre mitigations ...

On newer versions of MSVC, the compiler warns at `/Wall` if it will
generate a FENCE to mitigate against spectre

https://docs.microsoft.com/en-us/cpp/error-messages/compiler-warnings/c5045?view=vs-2019

This breaks testing, and is uninteresting for our purposes.

Check that including stdint.h works, because we don't include it on MSVC and it was failing previously.

Use c89 mode for gcc/clang.

MR: CMake fails if you use clang for c and gcc for c++.

Heyo, gaia?