[PollyACC] Configure PollyPPCG only if needed.

The PollyPPCG library is only needed when POLLY_ENABLE_GPGPU_CODEGEN=ON.
If disabled, the library target is still created, but not linked against
anything.

This change does not add create the PollyPPCG build target if not
needed.

Motivated by llvm.org/PR50021