cerbero: use relative paths for CMAKE_INSTALL_<dir>
This commit is a partial revert of 88c72a81.
According to the guide[1] of cmake, CMAKE_INSTALL_<dir>
has relative path
so it will be expanded to $(prefix)/$(CMAKE_INSTALL_<dir>)
.
[1] https://cmake.org/cmake/help/latest/module/GNUInstallDirs.html
Edited by Justin Kim