Use json_encode if available for generating map JS

OpenLayers map definitions can get very large, especially when using features on views layers. Pushing this all through drupal_to_js can get very costly -- this patch adds conditional usage of json_encode (PHP 5.2+) when available.

Patched against DRUPAL-6--2, but it's a very isolated change so should be easy to backport against 1.x, 0.x.