Changelog for 3.0-765

Fix broken build when using SDL from Externals.

The problem here was the logic that detects SDL in the main CMakeLists.txt
is not the same as it is in DolphinWX/CmakeLists.txt to set libraries. When
using SDL from Externals it failed at link time because -lSDL was never set.
This fixes the problem by using the same condition logic to set the libs
as used when detecting SDL in the first place.