Update CMakeLists.txt

[h1z1]

SDL build defaults to ON but can fail to be detected at build. examples/imgui_impl_sdl.cmake will fail as a result.

Option comment implies the build would succeed only if detected. Make it a fatal error for now.

[ 57%] Building CXX object CMakeFiles/imgui-sdl.dir/examples/imgui_impl_sdl.cpp.o
/tmp/build/imgui-cmake/examples/imgui_impl_sdl.cpp:51:17: fatal error: SDL.h: No such file or directory
 #include <SDL.h>
                 ^
compilation terminated.
[ 71%] Built target imgui_example_glut_opengl2
[ 85%] Built target imgui_example_null
make[2]: *** [CMakeFiles/imgui-sdl.dir/examples/imgui_impl_sdl.cpp.o] Error 1
make[1]: *** [CMakeFiles/imgui-sdl.dir/all] Error 2
make: *** [all] Error 2
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