[Development] HEADS UP: OSX build requirements for 5.7

[Tim Blechmann]

>> Does anyone see any issues with this going forward?
> 
> We need to be sure Qt compiles with the latest Xcode available on each of the 
> OS X versions that Qt can be developed on. Since 10.8→10.11 updating is free, 
> the only extra test is 10.7.

we have seen funny toolchain bugs, with 10.10 sdk and 10.8 deployment
target on 10.8, where std::exceptions could not be caught ... compiling
against 10.8 sdk solved that issue. so an xcode/sdk update is not
necessarily a no-brainer and there might be a benefit to still compile
with different xcode versions against different sdks.

otoh, ccache is an amazing way to reduce CI load :)

cheers,
tim