The --destdir is like automake's "make install DESTDIR=$TMP/blah" feature. But using configure --prefix should change what register --gen-script produces, so if that's not happening then it's a bug I think.
Perhaps I should tell you what I am about to do. I want to register some cabal packages in the nix software distribution system. a -> b Package a depends on b and c ` c Right now when a is build nix does create a new package databse registering b and c. I think this can be done better by letting b and c have its own package database containing only their libs of their own. Then when a has to be build GHC_PACKAGE_DATABASE=<path to pkg-db a>:<path to pkg-db b> can be used to tell ghc where to find those libs. Thus my trouble is that I should copy the lib to /nix/store/<somepath>/ Currently I'm using sed to prefix the right prefix using --gen-script .. Fixing cabal (if there is something to fix) does'nt have high priority for me right now. And in the long run I think we should think about writing something similar to scons (python build system) because some features should be written anyway (such as module dependency tracking ... ) At the moment I'm not sure how this should look like in detail. Thanks Marc