Rewrite flake

This copies most of the derivation from nixpkgs.  The old override
technique was problematic when development diverged since the last
release and the derivation in nixpkgs was not up to date any longer.

flake.nix

@@ -13,26 +13,49 @@
     version = head (match ".*version=([0-9.]*)\n.*" (readFile ./nvimpager))
               + "-dev-${toString self.sourceInfo.lastModifiedDate}";
     withoutDarwin = filter (s: !nixpkgs.lib.strings.hasSuffix "-darwin" s);
+    nvimpager = {
+      stdenv, neovim, ncurses, procps, scdoc, lua51Packages, util-linux
+    }:
+    stdenv.mkDerivation {
+      pname = "nvimpager";
+      inherit version;
+
+      src = self;
+
+      buildInputs = [
+        ncurses # for tput
+        procps # for nvim_get_proc() which uses ps(1)
+      ];
+      nativeBuildInputs = [ scdoc ];
+
+      makeFlags = [ "PREFIX=$(out)" "VERSION=${version}" ];
+      buildFlags = [ "nvimpager.configured" "nvimpager.1" ];
+      preBuild = ''
+      patchShebangs nvimpager
+      substituteInPlace nvimpager --replace ':-nvim' ':-${neovim}/bin/nvim'
+      '';
+
+      doCheck = true;
+      nativeCheckInputs = [ lua51Packages.busted util-linux neovim ];
+      # filter out one test that fails in the sandbox of nix
+      preCheck = let
+        exclude-tags = if stdenv.isDarwin then "nix,mac" else "nix";
+      in ''
+      checkFlagsArray+=('BUSTED=busted --output TAP --exclude-tags=${exclude-tags}')
+       '';
+    };
   in ({
     overlays.default = final: prev: {
-      nvimpager = prev.nvimpager.overrideAttrs (oa: {
-        inherit version;
-        src = ./.;
-        buildFlags = oa.buildFlags ++ [ "VERSION=${version}" ];
-        checkPhase = ''
-          runHook preCheck
-          make test BUSTED='busted --output TAP --exclude-tags=nix'
-          runHook postCheck
-        '';
-      });
+      nvimpager = final.callPackage nvimpager {};
     };
   } // (eachSystem (withoutDarwin defaultSystems) (system:
   let
-    stable = import nixpkgs { overlays = [ self.overlays.default ]; inherit system; };
-    nightly = import nixpkgs { overlays = [ neovim.overlay self.overlays.default ]; inherit system; };
+    callPackage = (import nixpkgs { inherit system; }).callPackage nvimpager;
+    neovim-nightly = neovim.packages.${system}.default;
+    default = callPackage {};
+    nightly = callPackage { neovim = neovim-nightly; };
   in {
-    apps.default = flake-utils.lib.mkApp { drv = stable.nvimpager; };
-    packages.default = stable.nvimpager;
-    packages.nightly = nightly.nvimpager;
+    apps.default = flake-utils.lib.mkApp { drv = default; };
+    packages = { inherit default nightly; };
   })));
 }