This fixes issues with ol.source.Vector being unable to use ol.ext.rbush,
but it also means that potential future external modules without node
module loader support will not work. If we ever depend on such a module,
we can replace "var define;" with a minimal AMD define shim.
CommonJS style modules will either assign `exports` to `module` or will assign properties to `exports`. This allows us to work with either style:
module.exports = Foo;
or
exports.Foo = Foo;
Support for additional properties that are assigned to `exports` (or non-function values assigned to `module.exports`) will likely take additional typedef work in the wrapper.
