Merge branch 'feature/avoid-unnecessary-rebuilds-without-fortran' into 'master'

[cmake] Avoid unnecessary rebuilds without fortran

When CMake doesn't detect Fortran, it writes an empty `FC.h` file, which gets included by `config.h`. This file is regenerated on each cmake run, causing continuous full rebuilds.

This merge requests fixes the issue by not rewriting the file if is already present.

See merge request !66