Don't define MAC_OS_X_VERSION_MIN_REQUIRED on iOS

Lack of the define is commonly used (and recommended by Apple) as a way
to distinguish iOS from OS X.

Task-number: QTBUG-35888
Change-Id: I5f3327e001c0af630984fbfde780d9b0c3e30754
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>