Fixed build process and dsdccx

Fixed build process to make mbelib and SerialDV optional effectively

It is now possible to compile dsdcc if only one, both or none of the mbelib or SerialDV dependencies are met. This solves issue #3

Of course without support of mbelib nor SerialDV no audio will come out. However this is a provision for future developments for use cases where only the MBE frames are needed (for example in a repeater operation).

Fixed missing process of DMR slot2 by SerialDV

All is in the title.