libopenmpi-dev dependencies on gfortran

[Valentin Haudiquet]

Hello!

I wanted to cross-compile packages depending on the OpenMPI library, but 
got into an error.

That error came from the dependencies of libopenmpi-dev on gfortran:
```
libopenmpi-dev
  |Depends: gfortran-15
   Depends: <gfortran-mod-16>
     gfortran
     gfortran-15
```

If I want to cross compile on an amd64 computer, for a riscv64 target: 
my package will need libopenmpi-dev:riscv64, which depends on 
gfortran-15:riscv64. But I also need gfortran-15:amd64 to do the 
compilation itself if my program is in fortran. Even if my program is 
not in fortran, gfortran-15:riscv64 conflicts with gcc-15:amd64, so it 
does not work.

I'm just trying to understand why a library has a runtime dependency on 
a compiler? That does not make much sense to me, but maybe there is a 
specific reason? Or is that a 'bug' and it should be a recommends/suggests?

Thank you for your work and your time,

--
Valentin Haudiquet
<valentin.haudiquet at canonical.com>