This isn't going to work if `rr` is resolved from `$PATH` (my setup), AFAICT.  That's a deal-breaker for distro packages.

I spent a fair amount of time a while back thinking about a run-time hack that works across all the setups we care about and pretty much drew a blank.  One thing we can do is have a build-time `-DRR_LIB_DIR` or something like that, that we can set to `$objdir/lib` in the CMakeList.  Then we check `ifdef RR_LIB_DIR` and then add that to `LD_LIBRARY_PATH` or prepend it to the preload lib path here.  Distro-package configs would somehow need to force off that -D.

Or something.

At any rate, let's not merge this until we have a plan that works for distro packages too.