If compiling for GP2X with SDL, make it use SDL 1.2 interface like the others

Doing this way because I have no idea if the GP2X port was SUPPOSED to use SDL or not in the first place

src/Makefile.cfg

@@ -410,6 +410,12 @@ endif
 endif
 endif
 
+ifdef GP2X
+ifdef SDL
+	SDL12=1
+endif
+endif
+
 ifdef ARCHNAME
 	OBJDIR:=$(OBJDIR)/$(ARCHNAME)
 	BIN:=$(BIN)/$(ARCHNAME)