[Nix-dev] $out in configureFlags is not expanded

[Peter Simons]

A change to the semantics of 'configureFlags' has significant potential
to break existing builds. Why would we take that risk? What exactly do
we gain? 

Personally, I've rarely had the need to pass $out in configureFlags, and
if I do need that, then

  preConfigure = ''configureFlags="--foo=$out/etc/foo.cfg"'';

works just fine. Maybe it would be sufficient to document the current
behavior in more detail?

Just my 2 cents,
Peter