Variable to find the SDK directories on macOS

[brianquinlan]

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

[stuartmorgan-g]

@liamappelbe I'm assuming "Backlog" means this isn't on the short-term roadmap currently? This seems like a blocker for adoption in flutter/packages, because any hard-coded path is not guaranteed to work for anyone else, since Xcode can be moved or renamed.

Having a variable that ffigen will map to the value of xcode-select -p is the only way to make a config file that is portable to other developers/machines.

[liamappelbe]

Ok, I can bump the priority on this one