问题
Is it possible to download and build against Mac OS X 10.9 and 10.10 SDKs on OS X 10.11 using Xcode 7?
回答1:
Yes it is possible, the XcodeLegacy script is a popular way to automate this.
Or you can do it manually, you need to put MacOSX10.9.sdk (or a symlink to it) into /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs. Note you'll need to replace it every time you update Xcode, so the symlink is a good idea.
来源：https://stackoverflow.com/questions/34780323/download-mac-os-x-sdks-for-10-9-and-10-10-in-xcode-7