[FIX] fixed macosx version query

 * `QSysInfo::MV_10_9` is not available in older Qt versions, which leads to a compile error for Qt < 4.8.6
 * replaced QSysInfo with `Gestalt` a mac osx API to query system information
 * this is a intermediate fix, as Gestalt will be deprecated in a future osx version, at some point we need to switch to [`sysctl`](https://developer.apple.com/library/mac/documentation/Darwin/Reference/Manpages/man3/sysctl.3.html) but this requires a manual mapping from kernel version to mac osx version