emacsPackages.notdeft: improve build process of notdeft-xapian

Before, the upstream Makefile[1] is used, which does not work well
with nix:
- Hardening flags of nix are ignored.
- One binary of buildInput xapian is run at build time.

[1]: https://github.com/hasu/notdeft/blob/1b7054dcfc3547a7cafeb621552cec01d0540478/xapian/Makefile#L10

pkgs/applications/editors/emacs/elisp-packages/manual-packages/notdeft/default.nix

@@ -30,14 +30,23 @@ let
     pname = "notdeft-xapian";
     inherit version src;
 
-    sourceRoot = "${src.name}/xapian";
+    strictDeps = true;
 
-    nativeBuildInputs = [
-      pkg-config
+    nativeBuildInputs = [ pkg-config ];
+
+    buildInputs = [
       tclap
       xapian
     ];
 
+    buildPhase = ''
+      runHook preBuild
+
+      $CXX -std=c++11 -o notdeft-xapian xapian/notdeft-xapian.cc -lxapian
+
+      runHook postBuild
+    '';
+
     installPhase = ''
       runHook preInstall
 
@@ -66,6 +75,10 @@ melpaBuild {
      ${lib.optionalString withIvy ''"extras/notdeft-ivy.el"''})
   '';
 
+  passthru = {
+    inherit notdeft-xapian;
+  };
+
   meta = {
     homepage = "https://tero.hasu.is/notdeft/";
     description = "Fork of Deft that uses Xapian as a search engine";