Don't use pkg-config for the OS X build

This can make configure add unwanted dependencies
to Qt, for example a homebrew installed libgthread
which is not available on stock OS X installs.

Task-number: QTBUG-42367
Change-Id: Idfbc5e627afb1a97e22c1d4df7cbc5e86fe42b37
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@theqtcompany.com>
Reviewed-by: Simo Fält <simo.falt@digia.com>