Improve build sytem support for readline instead of editline

Changes:

- CPP variable is now `USE_READLINE` not `READLINE`

- `configure.ac` supports with new CLI flag

- `package.nix` supports with new configuration option

- `flake.nix` CIs this (along with no markdown)

Remove old Ubuntu 16.04 stop-gap too, as that is now quite old.

Motivation:

- editline does not build for Windows, but readline *should*. (I am
  still working on this in Nixpkgs at this time, however. So there will
  be a follow-up Nix PR removing the windows-only skipping of the
  readline library once I am done.)

- Per
  https://salsa.debian.org/debian/nix/-/blob/master/debian/rules?ref_type=heads#L27
  and #2551, Debian builds Nix with readline. Now we better support and
  CI that build configuration.

This is picking up where #2551 left off, ensuring we test a few more
things not merely have CPP for them.

Co-authored-by: Weijia Wang <9713184+wegank@users.noreply.github.com>

flake.nix

@@ -230,6 +230,15 @@
           }
         );
 
+        # Toggles some settings for better coverage. Windows needs these
+        # library combinations, and Debian build Nix with GNU readline too.
+        buildReadlineNoMarkdown = forAllSystems (system:
+          self.packages.${system}.nix.override {
+            enableMarkdown = false;
+            readlineFlavor = "readline";
+          }
+        );
+
         # Perl bindings for various platforms.
         perlBindings = forAllSystems (system: nixpkgsFor.${system}.native.nix.perl-bindings);