Allow disabling swapped-endian clients

The swapped-endian support in X is a massive amount of very rarely used code. Since it is so rarely used, and hand-written, it is quite likely to have serious bugs that have gone undiscovered. Some of these bugs may very well have security implications.

Therefore, I recommend allowing swapped-endian support to be disabled. Ideally, it would be disabled by default, and require a configuration option to turn on.