Don't require downstream projects to enable OpenMP

[rhornung67]

Is your feature request related to a problem? Please describe.

Currently, if a RAJA build is configured with OpenMP enabled, a downstream project using that build must also have OpenMP enabled. Otherwise, the downstream project build will fail. The reason is this line https://github.com/LLNL/RAJA/blob/develop/include/RAJA/config.hpp.in#L263

Describe the solution you'd like

Don't require a downstream project to have OpenMP enabled if building against RAJA with OpenMP enabled. This would be consistent with how other back-ends in RAJA are handled. For example, we don't require CUDA to be enabled in a downstream project when built against RAJA with CUDA enabled.

Describe alternatives you've considered

A clear and concise description of any alternative solutions or features you've considered.

Additional context

The conditional logic in the code linked above is likely a remnant from when RAJA had OpenMP enabled by default. In the recent past, this was changed so that OpenMP is treated the same as other back-ends.