samueldr: cole-h yeah there is something suspicious going on, I now get:
  prefix="/nix/store/r3yfc612jfhlg25al36fcvb8d8jki30k-xatlas-0.0-git"
  exec_prefix="/nix/store/r3yfc612jfhlg25al36fcvb8d8jki30k-xatlas-0.0-git"
even though I have:
  xatlas-pc-file-contents = ''
    prefix="${placeholder "out"}"
    exec_prefix="''${prefix}"
    ...
I'd expect the final output to be just literally `exec_prefix="${prefix}"`. I don't quite get how the file can render almost-correctly but still bash (?) seems to do the substitution...