xo-unit: + ./pkgs/xo-subsys.nix for nix CI

pkgs/xo-subsys.nix

@@ -0,0 +1,36 @@
+{
+  # nixpkgs dependencies
+  stdenv, cmake, catch2, # ... other deps here
+
+  # xo dependencies
+  xo-cmake,
+
+  # args
+
+  #   attrset for fetching source code.
+  #    { type, owner, repo, ref }
+  #
+  #   e.g. type="github", owner="rconybea", repo="cmake-examples", ref="ex1b"
+  #
+  #   see [[../flake.nix]]
+  #
+  #cmake-examples-ex1-path
+
+  # someconfigurationoption ? false
+} :
+
+stdenv.mkDerivation (finalattrs:
+  {
+    name = "xo-subsys";
+
+    src = (fetchGit {
+      url = "https://github.com/rconybea/subsys";
+      version = "1.0";
+      #ref = "ex1";
+      #rev = "c0472c9d7e4d2c53bfb977d3182380832fe96645";
+    });
+
+    cmakeFlags = ["-DCMAKE_MODULE_PATH=${xo-cmake}/share/cmake"];
+    #doCheck = true;
+    nativeBuildInputs = [ cmake catch2 ];
+  })