CMake: change pocl.icd location rules slightly for CMAKE_INSTALL_PREFIX
CMake: change pocl.icd location rules slightly for CMAKE_INSTALL_PREFIX

If install prefix == /usr, install pocl.icd into /etc, otherwise install
into {prefix}/etc

franz/pocl,Oblomov/pocl,pocl/pocl,franz/pocl,franz/pocl,jrprice/pocl,pocl/pocl,pocl/pocl,rjodin/pocl,rjodin/pocl,rjodin/pocl,Oblomov/pocl,jrprice/pocl,jrprice/pocl,pocl/pocl,rjodin/pocl,pocl/pocl,jrprice/pocl,Oblomov/pocl,franz/pocl,Oblomov/pocl,Oblomov/pocl,franz/pocl