Visual Studio Builds: Avoid Implicit Linking to SDL

Cogl, when built with the SDL winsys, will include the SDL headers when
Cogl-based programs are built, which causes the SDL's wrapper for main()
to be used on Windows, causing an implicit requirement that all Cogl-based
apps must link to SDL2.lib and SDL2main.lib.  Avoid this behavior by
defining SDL_MAIN_HANDLED in the CFLAGS of the sample and interactive test
programs