build: Use boost shared_ptr by default on Windows
build: Use boost shared_ptr by default on Windows

As discussed on lists:
http://groups.google.com/group/ocio-dev/browse_thread/thread/52d3a2d8f830f29e

AcademySoftwareFoundation/OpenColorIO,AcademySoftwareFoundation/OpenColorIO,AcademySoftwareFoundation/OpenColorIO,AcademySoftwareFoundation/OpenColorIO,AcademySoftwareFoundation/OpenColorIO