[nix-local-build] Support `--prefix`, `--datadir`, etc

[acfoltzer]

Both new-configure and new-build seem to silently swallow these options at the moment.

[Ericson2314]

Do these flags effect building or just installing? I vaguely recall with --enable-relocatable its all argv[0]-relative, but I'm not sure otherwise.

edit answer seems to be #3473

[ezyang]

They affect building because prefix paths will affect what RPATHs we bake into the libraries we produce, as well as any hardcoded paths via a Paths module.

[phadej]

I tagged this to high priority, as e.g. haddock uses data

[ezyang]

Surely less high priority than getting new-install working, though?