Clean up CMake code

There are a few issues that are mostly cosmetic when building clicks but would cause problems if someone wanted to make a distro package.

CMAKE_INSTALL_PREFIX should not be set from within the project. clickable takes care of that already.

For the files in push the install path should not be defined as absolute. Use relative paths instead.

Using ${CMAKE_INSTALL_LOCALEDIR}/share/locale is wrong since CMAKE_INSTALL_LOCALEDIR itself expands to share/locale. This happens to work since GnuInstallDirs was not included and this CMAKE_INSTALL_LOCALEDIR was not defined.