attempt to perform rebuild only on darwin

package.json

@@ -9,6 +9,10 @@
     "type": "git",
     "url": "git://github.com/andrew/node-sass.git"
   },
+  "scripts": {
+    "install": "node rebuild.js"
+  },
+  "gypfile": true,
   "engines": {
     "node": ">=0.6.18"
   },

rebuild.js

+var spawn = require('child_process').spawn;
+
+if (process.platform === 'darwin') {
+  spawn('node-gyp', ['rebuild']);
+}