OS X: Search for SDK based on deployment target (#14324)

When available, use CMAKE_OSX_DEPLOYMENT_TARGET instead of the host OS X
version to select the default SDK.  This makes sense because one should
use the SDK matching the deployment target.

Suggested-by: John Ralls <jralls@ceridwen.us>
Merge topic 'osx-find-sdk-for-deployment-target' into next

95f78e0 OS X: Search for SDK based on deployment target (#14324)