Forces using SDL 2.0.14. Upgrades the SDL external to that version. Adds
a message when switching to the external.
Fixes an error where input_common only links to SDL when SDL2_FOUND is
set, but externals/CMakeLists cannot set that variable to the required
scope. Switch to using ENABLE_SDL2, which we can use since we now
include the SDL source.
de69a5266f