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.