PS2: Add support for building against old SDK if PS2SDK_OLD envvar set.

This will allow compilation using the older SDK until the buildbot
PS2 toolchain SDK can be upgraded and we can confirm the newer SDK
builds are working correctly.

configure

@@ -2316,7 +2316,9 @@ case $_host_os in
 		CXXFLAGS="$CXXFLAGS -G2"
 		DEFINES="$DEFINES -D_EE"
 		DEFINES="$DEFINES -D__PLAYSTATION2__"
-		DEFINES="$DEFINES -D__NEW_PS2SDK__"
+		if test -z "$PS2SDK_OLD"; then
+			DEFINES="$DEFINES -D__NEW_PS2SDK__"
+		fi
 		;;
 	ps3)
 		# Force use of SDL and freetype from the ps3 toolchain