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.

Source/Core/DolphinWX/CMakeLists.txt

@@ -13,12 +13,18 @@ set(LIBS	core
 			${X11_LIBRARIES})
 
 if(SDL2_FOUND)
+	# Using shared SDL2
 	set(LIBS	${LIBS} ${SDL2_LIBRARY})
+else(SDL2_FOUND)
+	if(SDL_FOUND)
+		# Using shared SDL
+		set(LIBS	${LIBS} ${SDL_LIBRARY})
+	else(SDL_FOUND)
+		# Using static SDL from Externals
+		set(LIBS	${LIBS} SDL)
+	endif()
 endif()
 
-if(SDL_FOUND)
-	set(LIBS	${LIBS} ${SDL_LIBRARY})
-endif()
 
 if(LIBAV_FOUND)
 	set(LIBS	${LIBS} ${LIBAV_LIBRARIES})