Use module_exists() instead of function_exists()

In nodequeue_save() we have the code:

    if (function_exists('views_invalidate_cache')) {
      views_invalidate_cache();
    }

That will lead to errors when these conditions are met:

* Your site uses both nodequeue and views.
* One of your modules depend on nodequeue but not on views (also no indirect dependency).
* That module implements a DrupalWebTestCase.
* The test case will now fail.

Why?

The function_exists('views_invalidate_cache') call will return TRUE because your system has views enabled and hence views_invalidate_cache() will be executed. But views_invalidate_cache() will fail because it expects the database table {cache_views} to be present. The {cache_views} table will not be present when executing the test case because the test case uses a prefixed database and since the test case has not installed views there is no {cache_views} in the prefixed database.

If we use module_exists('views') instead we get the correct behaviour because test cases rebuilds the list of enabled modules.