[cmake] use GNUInstallDirs to get canonical paths
[cmake] use GNUInstallDirs to get canonical paths

Fix hard-coded share install on multiarch layouts e.g. used by Exherbo
where the prefix might be something like /usr/${arch} but arch independent
files should still end up in /usr/share.

krrishnarraj/clpeak,krrishnarraj/clpeak,krrishnarraj/clpeak