Use the CPack DragNDrop generator instead of the deprecated PackageMaker
tool to package CMake itself.  This provides an installation experience
that is more consistent with other products on OS X and allows users to
select the destination directory easily.  It also avoids installing
"/private/var/db/receipts/com.Kitware.CMake.*" receipts that must be
removed by "pkgutil --forget com.Kitware.CMake" before another version
of CMake can be installed.

The DragNDrop installer does not support a post-flight script, so drop
our configuration of it.  The cmake-gui has an option for installing
symbolic links to enable command-line use.  In practice users may simply
add "/Applications/CMake.app/Contents/bin" to their PATH instead.