[BUG] CMake build flag DISABLE_OPENMP doesn't actually disable OpenMP

[robertmaynard]

Describe the bug
The OpenMP package is searched for unconditionally on https://github.com/rapidsai/cuvs/blob/branch-25.10/cpp/CMakeLists.txt#L585 and therefore enabling DISABLE_OPENMP doesn't actually do anything

Steps/Code to reproduce bug
Build with DISABLE_OPENMP enabled, and run ldd on the generated libcuvs.so. You will still see an openmp dependency

[robertmaynard]

re-opening this as multiple cuvs source files call omp_get_max_threads, omp_get_num_procs, omp_get_num_threads, and omp_get_thread_num all which can't occur when DISABLE_OPENMP is set.