With the PURPOSE a project can state what the package in question
is used for in the project (as opposed to describing what the package
does in general).
TYPE can be one of OPTIONAL (default)
                   RUNTIME - not needed for building, only at runtime
                   RECOMMENDED - as OPTIONAL, but you should really have it
                   REQUIRED - cmake will fail if one of these is not found

This can be set using the new function set_package_properties()

Alex