Serac: Problem with OpenMP + CUDA

Nobody has claimed this yet.

In serac, we're running into compilation issues on OpenMP targets when CUDA is enabled. The actual error is:

```
g++: error: unrecognized command line option ‘-fopenmp=libomp’
```

It looks like this flag is coming from https://github.com/LLNL/blt/blob/develop/cmake/thirdparty/SetupOpenMP.cmake which tries to manually manage compilation flags of targets that use OpenMP.

In my experience, CMake's built-in OpenMP target works pretty well (and it does not have these issues), so I think it would be beneficial to try using that directly instead of duplicating the effort in BLT.

An example CMake project that uses CUDA + OpenMP together is available here:

https://github.com/samuelpmishLLNL/openmp_cuda_example_project

it shows how to optionally enable CUDA language support, how to install the library, and how to use it as a subdirectory from another library as well.

Start by reading cmake/thirdparty/SetupOpenMP.cmake and reproducing the CUDA plus OpenMP configuration shown in the linked openmp_cuda_example_project. Compare BLT's flag handling with CMake's built-in OpenMP target. Done means OpenMP targets configure and compile with CUDA enabled without the unsupported -fopenmp=libomp error.

Written by the indexing model from the issue text.