Fix the Mac build when using the Qt SDK

To fix the build of the core library override Qt's default
minimum target version of 10.6 because we need to use libc++
for chromium which is only available from 10.7.

Change-Id: I8896c4741d1c9b7bfe6b1c045704aeb2d5affd17
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>