CMakeLists.txt: Handle 'undefined' values returned by getconf
CMakeLists.txt: Handle 'undefined' values returned by getconf

This allows to avoid build failures when using qemu user mode for riscv64 [1]:
the corresponding bug in qemu [2] will get fixed but handling that here allows
to keep pocl buildable by old qemu versions.

[1]: https://bugs.launchpad.net/ubuntu/+source/pocl/+bug/1942895
[2]: https://bugs.launchpad.net/ubuntu/+source/qemu/+bug/1942884

Signed-off-by: Alexandre Ghiti <b753907268438079e8c5ba52e7d31767daa077e2@canonical.com>

pocl/pocl,pocl/pocl,franz/pocl,franz/pocl,franz/pocl,pocl/pocl,pocl/pocl,pocl/pocl,franz/pocl,franz/pocl