MSVC Builds: Don't Link to SDL Automatically

Remove #pragma directives that causes any applications that use Cogl to
link to the SDL libraries when Cogl was built with the SDL winsys.  This is
mainly due to the availability of both SDL-1.x and SDL-2.x support in the
SDL winsys, where different libraries are linked for SDL-1.x and SDL-2.x.

To avoid having to link to the SDL/SDL2 libraries when the application code
is not directly using SDL/SDL2, define SDL_MAIN_HANDLED in the CFLAGS so
that SDL's wrapper main() implementation will not be used when the
application is being built.

Reviewed-by: Neil Roberts <neil@linux.intel.com>