Setup Y.config.global only if no user global was provided

The goal behind this change is to allow for YUI to work on CSP regulated sites and environments like Chrome extensions by providing the `global` in the user configuration. Setting `Y.config.global` in the user configuration doesn't trigger the use of `Function` to get to the global object.

No idea how to test the side-effects of this change.

/cc @caridy @tripp