Asterisk 11 bug, Asterisk 12 & 13 support, linking order fix, and ./configurable

Hi Darren,

Sorry for the noise -- each time i think "I'm done for the night; this is good enough", I keep doing more work on the code :)  This time I'm done - I promise.

This version of app_swift fixes a little bug in Asterisk 11 and adds Asterisk 12/13 support.

It also adds a nifty "configure" script to generate the Makefile -- some platforms have had to manually edit the Makefile previously.

Finally, I have also added the linking order fix by rampa069

Well tested on all combinations of Asterisk 11/12/13 and Swift 5/6. Older versions of Asterisk expected to work just fine.