Add "/MP" compiler flag for MSVC to speed up build process.

[CreativeLau]

No description provided.

[HuguesDelorme]

For me the changes should just check if cmake generator is VisualStudio. If yes then straight add /MP to Mayo_CompileOptions cmake variable

[CreativeLau]

Definitely, it should be modified as your recommendation. I have changed the pull request.

[HuguesDelorme]

Addition of /MP should happen only when cmake generator is VisualStudio(ie cmake creates VisualStudio projects)
Otherwise this option will break build of other make systems(eg ninja, nmake, ...)

[CreativeLau]

Yes, the /MP will be appended only if the compiler is MSVC.