Fix compilation on CentOS 7 (missing link to libdl)

[arekfu]

Description

This PR fixes compilation on CentOS 7.

Fixes #2848.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 15) on any C++ source files (if applicable) (not applicable)
• I have followed the style guidelines for Python source files (if applicable) (not applicable)
• I have made corresponding changes to the documentation (if applicable) (not applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable) (not sure this is possible?)

[HunterBelanger]

Hi @arekfu, have you tried to see if using the CMAKE_DL_LIBS works instead of adding dl directly (see here) ? In theory this should allow it to play nice with all operating systems.

[arekfu]

Hi @HunterBelanger, thank you for this suggestion! I was actually not aware of CMAKE_DL_LIBS. Let me test this...

[paulromano]

I'm a little confused by this. OpenMC does not make any direct calls to dlopen or other functions from libdl, so it must be a transitive dependency from one of the libraries we explicitly depend on. In that case, it should be the responsibility of whatever library is actually using it to make sure that the dependency on libdl propagates through.

Also side note that CentOS 7 is EOL in six months ⌛

EDIT: looks like I spoke too soon. We do use dlopen for loading custom source libraries, in which case it is appropriate to be linking against libdl.

[arekfu]

Hi @paulromano, I am aware that time is ticking for CentOS 7, but I think that the fix is valid for other platforms, too. I'm more worried about the failing checks. Is that expected?

[paulromano]

@arekfu There is an issue with a new release of scipy that is causing failures in CI. We just merged another PR that restricts the version of scipy for now, and I've just updated your branch to incorporate that. Hopefully tests should be passing now 🤞

[paulromano]

Tests passing now. Thanks @arekfu!