Add -DSDL2 for Win32/OSX support (untested), print SDL2 usage on startup

The quake2 binary now gets -DSDL2 in the CFLAGS, so Win32/OSX can
use different #include paths accordingly.
This is also (ab)used to print which SDL version is used on startup.
Don't use this for anything else, use
#if SDL_VERSION_ATLEAST(2, 0, 0)
instead.

I haven't tested building on/for Win32 or OSX, there may be more
work to do.

Furthermore I added Copyright-Info about CalculateGammaRamp()
in refresh.c (it's from SDL2)