message: Make sure SDL was found when using it
diff: @@ -283,7 +283,7 @@ if (VSNRAY_ENABLE_QT5)
  set(COMMON_SOURCES ${COMMON_SOURCES} viewer_qt.cpp)
  endif()
  -if (VSNRAY_ENABLE_SDL2)
  +if (VSNRAY_ENABLE_SDL2 AND SDL2_FOUND)
  set(COMMON_HEADERS ${COMMON_HEADERS} viewer_sdl2.h)
  set(COMMON_SOURCES ${COMMON_SOURCES} viewer_sdl2.cpp)
  endif()
  