Load t8code_jll only if preferences say so

[benegee]

Loading t8code_jll unconditionally can cause some trouble when system's MPI should be used. For example, it will always load MPI jll packages, which might cause conflicts. In case of OpenMPI_jll also OPAL_PREFIX will be overwritten.

[ranocha]

Thanks! Can we have a test for this to avoid regressions in the future?

[JoshuaLampert]

We should probably do the same in P4est.jl, see also trixi-framework/P4est.jl#166.

[ranocha]

@benegee Since it appears that this is needed (e.g., for your multi-GPU tests), shall we already merge this PR and release a new version now so that you have a bit more time to add a new regression test for this?

[benegee]

I was hoping to get #143 done, but at the moment I am not sure how to do it properly.

I am fine with merging this PR though. What I realized in #143 is that Aqua test break because t8code_jll, a direct dependency, is not loaded anymore.

[ranocha]

What I realized in #143 is that Aqua test break because t8code_jll, a direct dependency, is not loaded anymore.

You could change the setup of Aqua.jl depending on the environment variable determining which test to run.