Improve handling of Fortran-related CMake flags

[dsarmany]

Description

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

🌈🌦️📖🚧 Documentation 🚧📖🌦️🌈
https://sites.ecmwf.int/docs/dev-section/multio/pull-requests/PR-289

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 33.00%. Comparing base (a146425) to head (23d86d8).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #289      +/-   ##
===========================================
- Coverage    33.00%   33.00%   -0.01%     
===========================================
  Files          409      409              
  Lines        43458    43458              
  Branches      1978     1976       -2     
===========================================
- Hits         14344    14343       -1     
- Misses       29114    29115       +1     

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