Enable OpenMP via the target

We need to standardise how we handle OpenMP (and maybe other) flags to match how they're handled in other ICON libraries we control. Using the targets feels right.

See libfortran-support!140 (merged) for more details and considerations, as this MR has a direct dependency on that.

Before libfortran-support!140 (merged): libfortran-support exposes OpenMP::OpenMP_Fortran as PUBLIC, so any library linking against fortran-support::fortran-support automatically inherits the OpenMP flags transitively. libiconmath's three component libraries (horizontal, interpolation, support) all publicly link fortran-support::fortran-support, so they got OpenMP "for free" without asking for it.

After libfortran-support!140 (merged): that transitive propagation stops. libfortran-support no longer advertises OpenMP to its consumers. Any library that actually needs OpenMP must now explicitly declare the dependency.

This MR introduces the explicit declaration for the three libiconmath libraries that use OpenMP.