@acowley, thanks for your PR! By analyzing the history of the files in this pull request, we identified @abbradar, @vcunat and @edolstra to be potential reviewers.

Doesn't this interact with the preFixup phase somehow?

It doesn't seem to:

$ cd $(nix-build --no-out-link '<nixpkgs>' -A ghostscript)
$ otool -L bin/gs
bin/gs:
	/nix/store/7pr4ir0cb0k5k0v1fb8lpx0zalsrcb4x-ghostscript-9.20/lib/libgs.dylib.9.20 (compatibility version 0.0.0, current version 0.0.0)
	/nix/store/6f6kw88dcq5cfl2dhzs9cyvkpp9vl2i9-Libsystem-osx-10.11.6/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1226.10.1)

My understanding is that the preFixup is making up for linking the library in the build directory, and at that point the lib doesn't have its own name fixed up.

I copied the approach used in zlib. It's also done in postInstall in various places in llvm. That said, it's done in installPhase in other packages, so there's no unanimous idiom.