It’s macOS! 
Okay, thanks! That’s helpful. So the only option for now is to propagate the BUILD_RPATH requirements myself?
As for the other part of the question: how do the other targets achieve this? I read on the RPATH page of the wiki:
When building a target with RPATH, CMake determines the RPATH by
using the directories of all libraries to which this target links.
Shouldn’t this also happen for my bar::bar target, or is it because the LC_ID_DYLIB, the install name, contains @rpath that this mechanism doesn’t kick in or something?
Thanks again!