QPID-4737: Fix cmake "make package"

"make package" was attempting to install files with absolute paths to the final
install location, rather than the temporary install dir.

Trivial 1 line fix: set(CPACK_SET_DESTDIR ON)

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1466895 13f79535-47bb-0310-9956-ffa450edef68