[Development] HEADS UP: OSX build requirements for 5.7

[Tor Arne Vestbø]

Hey guys!

To clear up some resources in our CI system we're moving to the 
following build process:

- Build Qt on the latest OS X with the latest Xcode against the latest 
SDK available.
- Test that build by running the auto-tests on each of the supported 
deployment targets, down to the minimum deployment target we support 
(which for the time being is 10.7).

This means that we can save build time by not building Qt on those lower 
OS X versions, just running the tests there. It also means we can start 
removing SDK ifdefs in our code, since Qt should always be built against 
the latest available SDK.

This is the approach recommended by Apple, and the one we've been 
applying to the iOS port in the past.

Does anyone see any issues with this going forward?

Tor Arne