nix build: xo-randomgen: build+install examples

pkgs/xo-randomgen.nix

@@ -1,9 +1,11 @@
 {
   # nixpkgs dependencies
-  stdenv, cmake, catch2,
+  lib, stdenv, cmake, catch2,
 
   # xo dependencies
   xo-cmake, xo-indentlog,
+
+  buildExamples ? false,
 } :
 
 stdenv.mkDerivation (finalattrs:
@@ -12,8 +14,13 @@ stdenv.mkDerivation (finalattrs:
 
     src = ../xo-randomgen;
 
-    cmakeFlags = ["-DCMAKE_MODULE_PATH=${xo-cmake}/share/cmake"];
+    cmakeFlags = ["-DCMAKE_MODULE_PATH=${xo-cmake}/share/cmake"]
+                 ++ lib.optionals buildExamples ["-DXO_ENABLE_EXAMPLES=on"];
+
+    inherit buildExamples;
+
     doCheck = true;
+
     nativeBuildInputs = [ cmake catch2 xo-cmake ];
     propagatedBuildInputs = [ xo-indentlog ];
   })