Visual Studio Builds: Avoid Implicit Link to SDL

Cogl, when built with the SDL winsys, includes the SDL headers in its
headers, which causes main() to be defined to SDL's main() wrapper on
Windows, which means that SDL2.lib and SDL2main.lib need to be linked to
every single program that links to Cogl with the SDL winsys.

Avoid this behavior by defining SDL_MAIN_HANDLED in the CFLAGS of the
sample and test programs, and let people know that this is the case.