Compiling projects on macOS with modern Xcode versions

Apple likes deprecating APIs and architectures. Given that Garry's Mod is stuck on 32 bits and Mac OS X 10.7 SDK for now, it makes our lives harder to compile modules for it. However, there is a way to to it on modern Xcode versions (confirmed on Xcode 10) without much hassle.

You need:

• A copy of the Mac OS X 10.7 SDK (there are a multitude of SDK versions here)
• Xcode (and its terminal tools installed)

After extracting the SDK, copy its resulting directory to /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs (or whatever directory is outputted by xcrun --sdk macosx --show-sdk-platform-path, followed by /Developer/SDKs).
Modify the Info.plist file inside the Mac OS X platform directory (/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform or xcrun --sdk macosx --show-sdk-platform-path) such that the key MinimumSDKVersion contains the value 10.7 instead.

And that's it. garrysmod_common will do the rest and try to automatically use this SDK.