Add protobuf sources to support building outside of cocoapods

[elliottwilliams]

v4.0.0 added a dependency on protobuf, and updated the podspec to declare this, but didn't update the xcodeproj itself. This broke Carthage support and prevents building from a clone of the repo.

This change:

• Adds a protobuf submodule checked out at v3.14.0
• Generates a protobuf.xcodeproj by running cmake -G Xcode -T buildsystem=1 -DCMAKE_OSX_SYSROOT=iphoneos -DCMAKE_OSX_ARCHITECTURES='$(ARCHS_STANDARD)' -DCMAKE_OSX_DEPLOYMENT_TARGET=11.1 protobuf/cmake
  • buildsystem=1: uses the legacy build system. protobuf specifies dependencies in such a way that cmake can't generate using the modern build system
  • CMAKE_OSX_SYSROOT=iphoneos: builds protobuf using the iOS SDK
  • CMAKE_OSX_ARCHITECTURES='$(ARCHS_STANDARD)': prevents cmake from settings ONLY_ACTIVE_ARCHS=NO, which causes simulator release builds of GTXiLib to fail
  • CMAKE_OSX_DEPLOYMENT_TARGET=11.1: matches the GTXiLib's deployment target
  • If you think it'd be useful, I can add a Makefile target to regenerate this project in the future
• Creates a GTXiLib.xcworkspace which includes both projects
• Configures the GTXiLib target to find headers from protobuf and link against libprotobuf

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Testing done:

• Builds in Debug and Release configurations for iOS and iOS Simulator (x86-64) targets
• All tests pass except for GTXOOPFunctionalTests, where every test fails with No target application path specified via test configuration
  • I'm not sure whether I caused this – if there's a way to verify these tests on master i'd be happy to :)
• pod spec lint . fails, but it also fails on master. not sure whether there's an issue in the podspec, or if it's something on my end?
• carthage build --use-xcframeworks --no-skip-current succeeds

[kalkwarf]

Perhaps I misunderstand the PR, but I don't see GTXiLib.xcworkspace committed, nor do I spot a rule for cmake to generate it.

I'd sure love for this fix to land, so let me know how I can help. :-)