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.

CMakeLists.txt

@@ -274,9 +274,9 @@ if (ENABLE_SDL2)
         target_link_libraries(SDL2 INTERFACE "${SDL2_LIBRARY}")
         target_include_directories(SDL2 INTERFACE "${SDL2_INCLUDE_DIR}")
     else()
-        find_package(SDL2 2.0.12)
+        find_package(SDL2 2.0.14)
 
-        if(SDL2_FOUND)
+        if (SDL2_FOUND)
             # Some installations don't set SDL2_LIBRARIES
             if("${SDL2_LIBRARIES}" STREQUAL "")
                 message(WARNING "SDL2_LIBRARIES wasn't set, manually setting to SDL2::SDL2")
@@ -286,10 +286,10 @@ if (ENABLE_SDL2)
             include_directories(SYSTEM ${SDL2_INCLUDE_DIRS})
             add_library(SDL2 INTERFACE)
             target_link_libraries(SDL2 INTERFACE "${SDL2_LIBRARIES}")
+        else()
+            message(STATUS "SDL2 2.0.14 or newer not found, falling back to externals.")
         endif()
     endif()
-else()
-    set(SDL2_FOUND NO)
 endif()
 
 # Install any missing dependencies with conan install