BLT OpenMP setup overwrites consumer options on `OpenMP::OpenMP_Fortran

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### Repeated BLT OpenMP setup overwrites consumer options on `OpenMP::OpenMP_Fortran`

A project configures Cray Fortran OpenMP offload by adding required usage requirements to CMake’s imported `OpenMP::OpenMP_Fortran` target:

```cmake
target_compile_options(OpenMP::OpenMP_Fortran INTERFACE
$<$:
-f openmp-force-usm
--rocm-path=/opt/rocm-7.2.1
-haccel=amd_gfx942>)

Later, importing CAMP with find_package(camp CONFIG REQUIRED) loads:

campConfig.cmake
-> BLTSetupTargets.cmake
-> thirdparty/BLTSetupOpenMP.cmake
-> find_package(OpenMP REQUIRED)

This second find_package(OpenMP) resets
OpenMP::OpenMP_Fortran’s INTERFACE_COMPILE_OPTIONS. The consumer’s
Cray accelerator flags disappear, leaving only -h omp.

Can BLTSetupOpenMP.cmake, avoid calling
find_package(OpenMP) when the required OpenMP::OpenMP_* targets already
exist, or preserve and append their existing interface properties when
refreshing discovery?

Start with thirdparty/BLTSetupOpenMP.cmake and the BLTSetupTargets.cmake path loaded by campConfig.cmake, then inspect the second find_package(OpenMP) call and the existing OpenMP::OpenMP_Fortran interface options. Reproduce the camp import with the shown Cray flags; done means repeated setup no longer removes consumer options and -h omp remains alongside them.

Written by the indexing model from the issue text.