Apple seems to move around it's framework headers. Currently, on my machine, they have paths like:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks/Foundation.framework/Versions/C/Headers/NSDictionary.h

previously they looked like:

/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Foundation.framework/Versions/C/Headers/NSDictionary.h

It would be nice to have a magic variable that can be used in ffigen.yaml so that you can write paths like this:

$SDK/MacOSX.sdk/System/Library/Frameworks/Foundation.framework/Versions/C/Headers/NSDictionary.h