Xcode: Find fallback SDK if one matching the OS version doesn't exist.

For example if one installs Xcode 4.6 on OS X 10.9, it doesn't contain
a 10.9 SDK, so fallback to the next newest version which, in this case,
happens to be a 10.8 SDK.

This fixes bug #14572.
Merge topic 'xcode-sdkroot' into next

5dfe9b7d Xcode: Find fallback SDK if one matching the OS version doesn't exist.