vwebp_sdl: fix the makefile.unix
The FLAGS_LIBS was dropped at some point.
There is still an SDL_Main / main problem though, but it's a
separate topic.
@@ -428,7 +428,7 @@ extras/vwebp_sdl: extras/webp_to_sdl.o
extras/vwebp_sdl: imageio/libimageio_util.a
extras/vwebp_sdl: src/libwebp.a
extras/vwebp_sdl: EXTRA_FLAGS += -DWEBP_HAVE_SDL
-extras/vwebp_sdl: -lSDL
+extras/vwebp_sdl: EXTRA_LIBS += -lSDL
$(OUT_EXAMPLES) $(EXTRA_EXAMPLES) $(OTHER_EXAMPLES):
$(CC) -o $@ $^ $(LDFLAGS)
