Make LibSignal work on macOS

LibSignal_on_macOS.md

LibSignal is not officially supported on macOS because it's not one of Signal's app platforms (Signal Mac app uses Electron). You'll have to build the Rust parts yourself instead of using the prebuilt ones.

1. Check out https://github.com/signalapp/libsignal repo.
2. Have Rust as well as clang, libclang-dev, cmake, make, protobuf-compiler, git installed, or set it up using bin/mac_setup.sh.
3. Build the checked out repo using swift/build_ffi.sh --release.
4. nano *.podspec
5. Add s.osx.deployment_target = '10.15' after s.swift_version = '5' in Podspec file.
6. pod lib lint --platforms=macos
7. In your project's Podfile:

platform :macos, '10.15'
target 'MyApp' do
  use_frameworks!

  # Pods for MyApp
  pod 'LibSignalClient', :path => 'path/to/downloaded-libsignal-directory'
  ENV['LIBSIGNAL_FFI_PREBUILD_CHECKSUM'] = 'checksum_that_is_attached_to_every_libsignal_release (tar.gz.sha256 file)'
end

8. pod install

Now you should have LibSignal working on macOS. Go into Xcode and build the project!