install cmake bits to DATADIR instead of LIBDIR

`thrust` and `cub` are architecture-independent header-only libraries, therefore their `*.cmake` bits should be placed under `/usr/share/cmake/` instead of `/usr/lib*/cmake/` (which is the architecture dependent `/usr/lib//cmake/` on Debian and derivatives).
Please switch from `CMAKE_INSTALL_LIBDIR` to `CMAKE_INSTALL_DATADIR`. This also avoids embedding an architecture-dependent path in the generated `thrust-header-search.cmake` and `cub-header-search.cmake`