Mac: Do not require Xcode command line tools

/usr/bin/xcrun is the tool to use for running build tools from the
current Xcode installation, without the need to install anything beside
it.

Task-number: QTBUG-38015
Change-Id: I32ba8bfac2f2c49e863fe8aeeecaf3df80afb8c8
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>
Reviewed-by: Niels Weber <niels.weber@digia.com>