[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.

Title changed from [cmakeAvoid unnecessary rebuilds without fortran to [cmake] Avoid unnecessary rebuilds without fortran

Added backport bug buildsystem labels

Milestone changed to DUNE 3.0.0

3 weeks and no complaints, so I will be merging this one.

mentioned in commit f04ac18e

Status changed to merged

Mentioned in commit f04ac18e