cmdutils: move "#undef main" from ffplay.c to cmdutils.h

On Windows/MinGW the SDL cflags re-define the main() function, which
results in a linking error if the define is not undeffed.

Since the addition of the SDL output device, SDL cflags are used also
for compiling ffmpeg and ffprobe, so we need to move this trick from
ffplay.c to a common header.

Fix trac issue #256.