emscripten-buildbot.sh: force -s USE_SDL=0

This avoids using Emscripten-provided SDL headers from its own
sysroot instead of the headers in our own include directory!

Reference emscripten-core/emscripten#17647

Author: icculus

build-scripts/emscripten-buildbot.sh

@@ -55,7 +55,7 @@ mkdir buildbot
 pushd buildbot
 
 echo "Configuring..."
-emconfigure ../configure --host=wasm32-unknown-emscripten --disable-assembly --disable-threads --disable-cpuinfo CFLAGS="-O2 -Wno-warn-absolute-paths -Wdeclaration-after-statement -Werror=declaration-after-statement" --prefix="$PWD/emscripten-sdl2-installed" || exit $?
+emconfigure ../configure --host=wasm32-unknown-emscripten --disable-assembly --disable-threads --disable-cpuinfo CFLAGS="-s USE_SDL=0 -O2 -Wno-warn-absolute-paths -Wdeclaration-after-statement -Werror=declaration-after-statement" --prefix="$PWD/emscripten-sdl2-installed" || exit $?
 
 echo "Building..."
 emmake $MAKE || exit $?