Ignore unknown pragma warnings when building without OpenMP

[hainest]

Oddly, these also show up when ENABLE_STATIC_LIBS=OFF and USE_OpenMP=ON.

Fixes #1231

[kupsch]

This is the simplest way to accomplish this, and what I'd say to do.

A slightly better way would be conditionally create a macro DYNINST_PRAGMA_OMP that expands to nothing when not openmp is not enabled and _Pragma(...) when enabled, but this would require more changes for the limited use case.

[hainest]

This is the simplest way to accomplish this, and what I'd say to do.

A slightly better way would be conditionally create a macro DYNINST_PRAGMA_OMP that expands to nothing when not openmp is not enabled and _Pragma(...) when enabled, but this would require more changes for the limited use case.

Yeah, a macro would likely cause issues down the road if/when more omp is used. I don't like hardcoding the flag names, but it's really the only way to do it in CMake.