Add NixOS environment for ocaml-c

ocaml-c/default.nix

@@ -0,0 +1,11 @@
+with import <nixpkgs> {}; {
+    cairoprobeEnv = stdenv.mkDerivation {
+        name = "cairoprobe-env";
+        buildInputs = [ stdenv
+                        gcc
+                        pkgconfig
+                        cairo
+                        SDL2
+                      ];
+    };
+}