Add ability to disable NCCL at runtime

[oschuett]

I'd like to build COSMA with NCCL for CP2K. However, NCCL requires that every MPI rank has a dedicated GPU. Since this is not always the case, it would be great if there was a way to disable NCCL at runtime.

See also cp2k/cp2k#2202.

[oschuett]

Gentle ping :-)