Allow to build with CLT instead of Xcode

Up until 6.8.1 I could build the base/svg/tools/translations modules using the Command Line Tools for Xcode and by setting -DCMAKE_OSX_DEPLOYMENT_TARGET to a valid value (min or bigger) when configuring qtbase.

But with 6.8.2 and due to commit 5e246b8af86d91252e49321d9c46897e22a4c797 the configure script is forced to exit if it doesn't recognize an Xcode version.
(If only CLT are installed then the version check returns an empty value).

Please, rework the configure scripts to allow builds with the CLT if Xcode is not available. Maybe check if the set macos SDK conforms to the minimum value required?

Alternatively ( ? )
In case you don't want to bother with any of this please fix the following.
I can configure+build+install qtbase if I pass -DQT_FORCE_WARN_APPLE_SDK_AND_XCODE_CHECK=ON.
However, this option isn't respected when I continue to build the qtsvg module with <installDir>/bin/qt-configure-module and the configuration fails.