macOS build instructions need updating for full client build

[dgelessus]

The "Building on macOS & Linux" instructions currently say:

Linux and macOS are not officially supported targets, and the game client does not compile. However, several of the tools can be built and run.

This should be updated to mention that macOS has a full client now.

While trying to guide someone through the macOS build process, I also noticed some other things that aren't mentioned in the instructions, and a few small errors:

• There seems to be a mistake in the CMake options: for the configure step, it should be -B instead of --builddir, and for the build step, it should be --build instead of --builddir. (This applies to Linux as well. I can't find a --builddir option documented anywhere.)
• The CMake command for the configure step needs -G Xcode to support building the Mac client. (This means that macOS and Linux need different CMake configure commands now.)
• The instructions don't mention what the minimum required/supported macOS version is. According to various places in the CMake build scripts, macOS 10.14 seems to be the minimum for building the client. The tools can probably built with much older versions.
• -DUSE_VCPKG=ON should probably be recommended as the default for macOS builds. At least PhysX is otherwise difficult/impossible to install via the usual macOS package managers (Homebrew, MacPorts).
• Some of the vcpkg dependencies require extra commands that are not part of the standard macOS developer tools. It should be mentioned somewhere that these need to be manually installed via a different package manager before trying to use vcpkg.
  • pkg-config is required by zlib (and probably others)
  • python3 is required by vcpkg-tool-meson (recent Xcode versions include some version of Python 3 I believe, but older ones don't)
  • nasm is required by libjpeg-turbo and libvpx (and possibly others, at least when building for x86_64)

[colincornaby]

According to various places in the CMake build scripts, macOS 10.14 seems to be the minimum for building the client.

10.14 is indeed the minimum version for running the client. (Note: building/running.)

I'm not sure what the minimum Xcode to build would be. I'd always encourage the latest - but we might go back to Xcode 11.