configure and mkspecs: Don't try to find xcrun with xcrun

Since Xcode 8 (beta 2) that tool is no longer available
through xcrun. We resort to xcodebuild instead.

Change-Id: If9d7b535c1cbac2caae0112b2003283aeff34fb9
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>