CMake: Do not specify absolute paths but relative ones.

[eolivi]

(important for packaging with cpack)

"Relative paths are already relative to CMAKE_INSTALL_PREFIX by default" http://cmake.3232098.n2.nabble.com/Getting-empty-package-with-CPack-td7589998.html

please note that I'm not a zlib contributor, so consider this PR as an advice only.

[bonimy]

Bumping this as useful. Was just dealing with this.