Add support for a new build option: useSourceMaps

This update adds support for a new boolean build option named `useSourceMaps`. It is applied when the Google Closure Compiler is enabled via the `layerOptimize` option.
* **true** - This is the default value.  Source maps will be generated.
* **false** - Source maps will not be created.

Dojo toolkit ticket: https://bugs.dojotoolkit.org/ticket/17425