set the PATH correctly on windows, using absolute paths to the mapnik…

… and zipfile libs - this actually works now and will obsolete the code at https://github.com/mapbox/tilemill-win-launcher/blob/f15a96cd1ba70ac38b5ecc1efa61f8562d6ccc0b/tilemill.cc#L209 - refs tilemill-project#1742

index.js

@@ -15,7 +15,9 @@ process.argv[0] = 'node';
 if (process.platform === 'win32') {
     // HOME is undefined on windows
     process.env.HOME = process.env.USERPROFILE;
-    process.env.PATH = "node_modules/mapnik/lib/mapnik/lib;node_modules/zipfile/lib;"+process.env.PATH;
+    // Add custom library paths to the PATH
+    process.env.PATH = path.join(__dirname,"node_modules/mapnik/lib/mapnik/lib") +
+        ";" + path.join(__dirname,"node_modules/zipfile/lib");
 }
 
 // Default --config flag to user's home .tilemill.json config file.