Fix: OpenMPI linkage

[sandro-elsweijer]

Closes #1795

Describe your changes here:
I have the problem, that I have to build t8code with OpenMPI .
This works as long as I link to a self-build SC, because the arising warnings are within SC and Werror of t8code does not trigger.
But when I try to build t8code with the shipped SC, the warnings are part of t8code and Werror will kick in.
Therefore, i implemented a check which determines if OpenMPI was used and then suppresses that the warning for SC (not for t8code) is treated as an error.
The code builds an example program which checks if this macro was defined.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.39%. Comparing base (9e88865) to head (d8aa6bb).
⚠️ Report is 81 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1794      +/-   ##
==========================================
- Coverage   74.51%   74.39%   -0.13%     
==========================================
  Files         100      101       +1     
  Lines       19104    19178      +74     
==========================================
+ Hits        14236    14268      +32     
- Misses       4868     4910      +42     

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[sandro-elsweijer]

Niklas and I discussed this topic and it seems there was a misunderstanding about this problem. Niklas proposed solution does not fix the problem. But I tried to use a FindMPI script which can control the MPI CXX interface via a variable, but this also did not work.
Handing it to you @Davknapp