Revert "drop USE_OPENMP flag, use _OPENMP instead, since it indicates compiler support"

This reverts commit 40ce6e8a977aeb185116419910e2c1fce17c450f.

Since 'nvcc' is the compiler for nbody.cu, it doesn't see the _OPENMP define
which was present in the source compilation of the OpenMP implementation. We
can't depend on the _OPENMP macro's existence to determine how the non-CUDA
portions were compiled.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>