CMakeList.txt fixes

The 'debug' target no longer includes the -Werror switch.

Added -Wincompatible-pointer-types to 'debug' and 'maintainer'
targets.
Removed 'const' from the argv argument in a2test.c

There's no consensus on whether or not main() should have a
'const' argv argument, and the SDL_main wrapper isn't helping,
apparently, so the #ifdef attepmt of dealing with it has been
removed. There will probably be warnings on some platforms no
matter what.