Exit if the FindSDL script finds 2.x instead of 1.x (#4948)

CMakeLists.txt

@@ -133,6 +133,9 @@ if(Qt5Core_FOUND OR FOX_FOUND)
   find_package("SDL")
   if(SDL_FOUND)
     message(STATUS "SDL Lib: ${SDL_LIBRARY}; Headers: ${SDL_INCLUDE_DIR}; Version: ${SDL_VERSION_STRING}")
+    if (NOT ${SDL_VERSION_STRING} VERSION_LESS "2.0")
+      message(FATAL_ERROR "OpenTX requires SDL 1.x, not 2.x")
+    endif()
   else()
     message(STATUS "SDL not found! Simulator audio, and joystick inputs, will not work.")
   endif()