Get `cabal ..` builds in the dev shell working again

flake.nix

@@ -36,6 +36,12 @@
       devShells = hslib.devShells {
         extraBuildInputs = pkgs:
           with pkgs; [
+            # We configure cabal to use zlib:pkg-config so we better supply
+            # pkg-config in the dev shell or `cabal ...` builds will surely
+            # fail.
+            pkg-config
+
+            # And also that pesky zlib dependency.
             zlib
           ];
       };