[CMake] Generating archives and binaries with different PREFIX settings using CPack

[Clifford Yapp]

I'm trying to generate both source tarballs and binary packages using
CPack, and I'm at something of a loss as to how to achieve the
following:

I want the binaries (RPM, DEB, etc.) to respect the
CMAKE_INSTALL_PREFIX.  I want the source tarballs to expand into their
directory like a standard tarball, without any prefix directories.  As
far as I can tell, both source and binary packaging structure is
governed by CPACK_PACKAGING_INSTALL_PREFIX - am I missing something
that will let me specify one setting for binary archives and another
for source archives?  (Ideal would be per-generator settings, so I
could generate a binary tar.gz bundle that would expand locally as
well, but the really important one is source tarballs.)

Thanks,
CY