On the host machine, register for a free Apple developer account, then download the SDK here.
Using Mac OS X, you can mount the dmg, and then extract the SDK with:
$ hdiutil attach Xcode_7.3.1.dmg
$ tar -C /Volumes/Xcode/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/ -czf MacOSX10.11.sdk.tar.gz MacOSX10.11.sdk
Clean up the files you don't need:
diskutil unmount /Volumes/Xcode
rm Xcode_7.3.1.dmg
Alternatively, you can use 7zip and SleuthKit to extract the files one by one. The script extract-osx-sdk.sh automates this. First ensure the dmg file is in the current directory, and then run the script.
You may wish to delete the intermediate 5.hfs file and MacOSX10.11.sdk (the directory) when you've confirmed the extraction succeeded.
apt-get install p7zip-full sleuthkit
contrib/macdeploy/extract-osx-sdk.sh
rm -rf 5.hfs MacOSX10.11.sdk
Copy it to the Gitian VM and clean up, e.g.:
# Create inputs folder and copy SDK
ssh gitian "mkdir -p gitian-builder/inputs" && scp MacOSX10.11.sdk.tar.gz gitian:gitian-builder/inputs
# Cleanup
rm MacOSX10.11.sdk.tar.gz
See README_osx.md for troubleshooting tips.