Revert "SDL: make the default NO_SDL"

Now #397 is merged, this PR proposes making the default behavior to _not_ include SDL where it can be included by running 

    make GCS_QMAKE_OPTS="CONFIG+=SDL" -j7 gcs

previously a few developers expressed a preference for doing this as it greatly simplifies setting up a development environment since SDL is clunky to install.  Ideally we would then replace USB joystick support with something that can be integrated into our source tree.

This is also consistent with how we include other optional features like OSG.  If the issue is that some people want to always have SDL/OSG included we should be able to pick up environmental variables (or they can set add it in Qt Creator under the qmake options).