OS X: SDK configuration

Adds several improvements for setting the SDK.

Change-Id: Id6aa7c972ec5f11f1a43ced90f0411e55328eef3
Reviewed-on: https://gerrit.libreoffice.org/9952
Tested-by: LibreOffice gerrit bot <gerrit@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>