OpenMP tests fail when ZFP_WITH_OPENMP and ZFP_WITH_HIP are both enabled

[lindstro]

We are still investigating this issue, first reported on #85, which appears to be due to -fopenmp not being passed and _OPENMP not being defined when src/zfp.c is compiled. This option is passed when compiling the tests, however.

[GarrettDMorrison]

Fixed in the staging branch, will close on merge into develop