build: prevent SDL2 from polluting global cflags and extralibs

Remove the SDL_main define from the global cflags but not from the
ffplay cflags, and the -mwindows linker option from extralibs instead
of overriding it with the addition of -mconsole.

Reviewed-by: wm4 <nfxjfg@googlemail.com>
Signed-off-by: James Almer <jamrial@gmail.com>