cmake: Use SDL 2.0.14 and fix CMake scope issue

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.

1 files changed, 0 insertions, 0 deletions

diff --git a/externals/SDL b/externals/SDL
-Subproject 983bbf9ef3e572a073a6f5877faf1c0b4803527
+Subproject 4cd981609b50ed273d80c635c1ca4c1e5518fb2