xo-indentlog: build: XO_ENABLE_DOCS for docs build

pkgs/xo-indentlog.nix

@@ -1,8 +1,15 @@
 {
   # dependencies
-  stdenv, cmake, catch2,
+  lib, stdenv, cmake, catch2,
+
+  python3Packages,
+
+  doxygen, sphinx, graphviz,
 
   xo-cmake,
+
+  buildDocs ? false,
+  buildExamples ? false,
 } :
 
 stdenv.mkDerivation (finalattrs:
@@ -12,7 +19,29 @@ stdenv.mkDerivation (finalattrs:
 
     src = ../xo-indentlog;
 
-    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 ];
+
+    postBuild = lib.optionalString buildDocs ''
+      cmake --build . -- docs
+    '';
+
+    nativeBuildInputs = [ cmake
+                          catch2
+                          xo-cmake ]
+                        ++ lib.optionals buildDocs [
+                          doxygen
+                          sphinx
+                          graphviz
+                          python3Packages.sphinx-rtd-theme
+                          python3Packages.breathe
+                          python3Packages.sphinxcontrib-ditaa
+                          python3Packages.sphinxcontrib-plantuml
+                        ];
   })