Use cmake -B and -S flags to specify the build path

[Atry]

According to https://cmake.org/cmake/help/latest/manual/cmake.1.html , -B and -S flags are the right way to specify the build path

[fredemmott]

• I'm not sure this is 'recommended' as such - it's a way, but the way we're doing things is still documented, and (sorry if I missed it) I'm not seeing a deprecation notice or similar
• Unless there is a clear recommendation, I prefer the existing syntax: it's compatible with cmake --build and cmake --install, but -S and -B appear to be specific to the configure stage. For context, these are useful when using a backend other than make - for example, xcode projects or ninja. It will run make, ninja, msbuild, xcbuild, or whatever it was configured with, with consistent arguments. This is very handy for cross-platform CI.

I'm not certain on those two points, but regardless, we shouldn't do this yet: we still support Ubuntu 18.04 (cmake 3.10) and Debian Buster (cmake 3.13), and these options appear to be new in CMake 3.14.

• from a user perspective, I think we should aim for as few version-specific instructions as possible
• from our perspective, applying this to our own build system would require forking the packaging scripts between versions

[Atry]

How about add the -B and -S flags to macOS build? Homebrew should have installed the latest cmake.

[fredemmott]

That's true, however:

• there's still not an indication it's preferred
• I'd rather us avoid unnecessary differences between any of our platforms; keeping stuff the same makes it easier to focus on what's actually different and potentially behavior-changing

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