Fix CMake warning CMP0177

This fixes a the CMake warning CMP0177. CMake used to not allow relative paths in the install() directive. Now they normalize the paths so this is allowed, however, now they throw a warning for old code if they notice un-normalized paths. This MR fixes this by simply using the path relative to the install prefix, instead of calculating it from the binary directory.

In case of being accepted, I would suggest to also back-port it to 2.10.