Also search for dylibs in case we are on Darwin

[luke-clifton]

• [x] Tested on an x11 machine
• [x] Compilation warnings were addressed
• [ ] cargo fmt has been run on this branch
• [x] cargo doc builds successfully
• [ ] Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• [ ] Updated documentation to reflect any user-facing changes
• [ ] Created or updated an example program if it would help users understand this functionality

I have a use case where I want to run X11 applications on Darwin (using Xquartz). I've tested this change on Linux and MacOS.

I didn't introduce a conditional here, though I can if that is preferred, just let me know.

I did not run cargo fmt on this branch because it made changes elsewhere in the code. The part I added is unchanged by cargo fmt.