GCS Control: Make SDL optional

To compile with SDL joystick input support compile GCS with
make GCS_QMAKE_OPTS="CONFIG+=SDL" -j7 gcs
GCS Control: Remove commented code
Fix for excluding SDL
GCS Controller: Add an option to control via GCS Receiver instead
GCS Control: Update the GCS Receiver object
GCS: Also exclude sdlgamepad library when unused
SDL: Mask out more includes
SDL: Only copy the dll when used
Path to turn on compiling SDL by default.

Turn off SDL with: make GCS_QMAKE_OPTS="GCS_NO_SDL=NO" gcs
Merge pull request #440 from kubark42/gcs-control-via-gcsreceiver

GCS Control via GCSReceiver

Ok.  I conceed to the majority.