How to get 10.8 SDK and 5.1 compiler in Xcode 6
It's probably not supported by Apple, but I've seen good results by copying the 10.8 SDK from the Xcode 5.1.1 DMG.
sudo cp -a /Volumes/Xcode/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.8.sdk \ /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs
I tried different ways, and it seems that both @trojanfoe and @byronc are right. On the one hand, by targeting 10.8 with the 10.9 SDK, all the compiling stuff ends fine, so it seems to me the most correct way, using "official" Apple stuff. On the other, when I tried to copy and paste the 10.8 SDK from an old Xcode 5 installation to the Xcode 6 correct folder, the SDK showed up in my project, so this way seems functional too, even if I think it is not the most clear way.