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 33.51%. Comparing base (3395b02) to head (5199f1f).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #289   +/-   ##
========================================
  Coverage    33.51%   33.51%           
========================================
  Files          379      379           
  Lines        42648    42648           
  Branches      1839     1839           
========================================
  Hits         14292    14292           
  Misses       28356    28356           

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