[CMake] CPack. Modifying the installation prefix when building relocateable RPMs. v3.0.1

[Alan.Thompson at glasgow.ac.uk]

I am trying to build rpms for projects so they can be installed together
using a modified yum into a user defined area as
<new_prefix>/<project>/<platform>/<installed_binaries>
hence the rpms should look like
<relocatable>/<project>/<platform>/<installed_binaries>

The only way I could find to do this was to
set(CPACK_PACKAGING_INSTALL_PREFIX <relocatable>)
set(CPACK_INSTALL_PREFIX <relocatable>/<project>/<platform>)
and then call an install script which simply does
SET(CPACK_SET_DESTDIR "")

This doesn't make make much sense and I feel I must have missed 
something
out particularly as I am using version 3.0.1 and the above is not
necessarily compatible with other versions.
I'd be grateful for any advice or a more sensible approach
Thank you
Stan Thompson