[CMake] still having a problem with PackageMaker, and now DEB

[Chris Wolf]

Ok, I finally figured this out.  I needed to set set(CPACK_SET_DESTDIR true)
so that CMAKE_INSTALL_PREFIX overrides the default prefix of "./usr" baked into
PackageMaker and DEB generators.

CPACK_PACKAGING_INSTALL_PREFIX is a total "red herring" - who knows what
that's for? ;)

BTW, I led to the solution when I find this archived post:
http://www.mail-archive.com/cmake@cmake.org/msg16195.html

...which exactly confirmed by observation of the default behavior
of PackageMaker and DEB generators.