nix build: xo-reader: build + install docs+examples + build fix

pkgs/xo-reader.nix

@@ -1,8 +1,15 @@
 {
   # dependencies
-  stdenv, cmake, catch2, replxx,
+  lib, stdenv, cmake, catch2, replxx,
 
-  xo-cmake, xo-expression, xo-tokenizer
+  python3Packages,
+
+  doxygen, sphinx, graphviz,
+
+  xo-cmake, xo-expression, xo-tokenizer,
+
+  buildDocs ? false,
+  buildExamples ? false,
 } :
 
 stdenv.mkDerivation (finalattrs:
@@ -12,11 +19,35 @@ stdenv.mkDerivation (finalattrs:
 
     src = ../xo-reader;
 
-    cmakeFlags = ["-DCMAKE_MODULE_PATH=${xo-cmake}/share/cmake"];
+    cmakeFlags = ["-DCMAKE_MODULE_PATH=${xo-cmake}/share/cmake"]
+                 ++ lib.optionals buildDocs ["-DXO_ENABLE_DOCS=on"]
+                 ++ lib.optionals buildExamples ["-DXO_ENABLE_EXAMPLES=on"];
+
+    inherit buildDocs;
+    inherit buildExamples;
+
     doCheck = true;
-    nativeBuildInputs = [ cmake catch2
-                          xo-cmake
-                          xo-expression
-                          xo-tokenizer
-                        ];
+
+    propagatedBuildInputs = [ ];
+
+    postBuild = lib.optionalString buildDocs ''
+      cmake --build . -- docs
+    '';
+
+    nativeBuildInputs = [ cmake
+                           catch2
+                           xo-cmake
+                           xo-expression
+                           xo-tokenizer
+                         ]
+    ++ lib.optionals buildExamples [ replxx ]
+    ++ lib.optionals buildDocs [
+      doxygen
+      sphinx
+      graphviz
+      python3Packages.sphinx-rtd-theme
+      python3Packages.breathe
+      python3Packages.sphinxcontrib-ditaa
+      python3Packages.sphinxcontrib-plantuml
+    ];
   })