This commit fixes a regression introduced in r28000 (COMP: Update build
system removing support for C++98/Qt4/VTK7).

Following commit r28000 the ivar "required_deployment_target" was never
set because the module SlicerInitializeOSXVariables CMake  module is
included from the top-level CMakeLists.txt before the "project()" statement
and in that case Slicer_SOURCE_DIR was not defined too.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@28009 3bd1e089-480b-0410-8dfb-8563597acbee