Fix package config missing the OpenMP dependency

What is the bug

Consuming an OpenMP-enabled iconmath via find_package(iconmath) fails at the consumer's configure time:

CMake Error: The link interface of target "iconmath::support" contains:
    OpenMP::OpenMP_Fortran
but the target was not found.

Since !93 (merged) the Fortran libraries link OpenMP::OpenMP_Fortran as a target. The links are PRIVATE, but for static libraries they are still exported in the link interface (as $<LINK_ONLY:...>), and CMake requires the imported target to exist when the consumer loads iconmath-targets.cmake. The generated package config never declares OpenMP, so consumers that have not found OpenMP themselves cannot load the package at all.

How do you fix it

Add find_dependency(OpenMP REQUIRED COMPONENTS Fortran) to cmake/config.cmake.in, mirroring the find_package call in the top-level CMakeLists.txt. It is nested inside the existing IM_ENABLE_FORTRAN guard and additionally guarded by IM_ENABLE_OPENMP, so OpenMP-off and C++-only builds are unaffected. No location hint is needed: OpenMP is resolved by compiler introspection, not a package directory.

How urgent is the bugfix

• I need it as soon as possible
• I can wait for a couple of days
• None of my current codes is directly affected

Mandatory steps before review

• Gitlab CI passes (Hint: use make format for linting)
• Bugfix is covered by additional unit tests
• Mark the merge request as ready by removing Draft:

Mandatory steps before merge

• Reviewed by a maintainer
• Incorporate review suggestions
• Remember to edit the commit message and select the proper changelog category (feature/bugfix/other)

You are not supposed to merge this request by yourself, the maintainers of libiconmath take care of this action!