Big new SDL2 merges (please help test and update ports if you can!)

[davidjoffe]

Hi, have just done a big merge of Matteo Bini's implementation of updating Dave Gnukem from LibSDL1.2 to SDL2 (plus a lot of other small changes):

#140

Would be great to let this get a lot more testing - and port maintainers please report issues (either for discussion here or perhaps at the 'issue' item above) or anything else related requiring discussion ..

Downstream port maintainers will probably need to do a little work to get things running again - I hope there are no major conflicts etc. with makefiles etc.