[CMake] Defined extern/ headers as system for MSVC, removing warnings
[CMake] Defined extern/ headers as system for MSVC, removing warnings

- Warnings still do happen in .cpp when compiling though; added details to know what they really are

- A general variable is now set instead of separate ones
    - Elements are unquoted so that the list gets more readable
    - Replaces ';' characters, introduced with that way of doing it, by spaces

Razakhel/RaZ,Razakhel/RaZ