Fix macOS build

[ericwa]

This fixes the Xcode project build on my macOS 10.15 (x86_64) system.

Nothing outside the MacOSX directory is modified, so it shouldn't break anything else.

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The main issues I had to deal with were:

• we needed a build of zlib
• the QuakeSpasm libopus.dylib was built without the encoder, which was needed by snd_voip.c. So we need to rebuild that with the encoder.

To address these, I went with using vcpkg to build static libs for these dependencies (see setup-vcpkg.sh).