Do not provide an AMD environment to ol.ext modules

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.

tasks/build-ext.js

@@ -50,6 +50,7 @@ function wrapModule(mod, callback) {
         '(function() {\n' +
         'var exports = {};\n' +
         'var module = {exports: exports};\n' +
+        'var define;\n' +
         '/**\n' +
         ' * @fileoverview\n' +
         ' * @suppress {accessControls, ambiguousFunctionDecl, ' +