Add --with-sdl-net option to enable network builds

[jbaicoianu]

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Adds ./configure --with-sdl-net option to enable networking support with Emscripten builds

[dreamlayers]

It should now be possible to build with SDL_net using emconfigure ./configure CFLAGS="-O3 -s USE_SDL=2 -s USE_SDL_NET=2" CXXFLAGS="-O3 -s USE_SDL=2 -s USE_SDL_NET=2". This makes the normal detection work. Note that the C compiler is used for configure tests, and the C++ compiler is used for building, so both flags need to be altered. Also note that the default -O3 would be lost if not put here. A more convenient way would be nice, but I'm not sure about this here.

I've cherry-picked df3b102. Thank you.