1. First of all, this doesn't work in nixpkgs. Per [1], gcc ignores `-L`
   for purposes of `--print-file-dirs`, which breaks horribly on linux. But
   if we don't pass extra dirs, meosn first just tries `-l...`, which does
   work.

2. Even if it did work, `libdir` means where we are installing libs, not
   where libs are expected to be found. Those are not necessarily the
   same (again, nixpkgs), and even when they are and non-standard, it is
   better to use DESTDIR or have a modified toolchain.

[1]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87758

This means do autodetection if the user doesn't specify.

  1. Fix that for libsodium

  2. Consistently capitalize

 - `crossSystem` in `shell.nix`
 - `buildInputs` vs `nativeBuildInputs`

Fix build in nix shell

This commit just moves the code, nothing leverages this yet.

Also heh, from editline to libeditline again, after
a142164.

 - editline_dep for editline.h

 - libbz2_dep for bzlib.h, except we cannot because no pkg-config file
   so document that.