Revert "sdl2.m4: Deprecate AM_PATH_SDL2 in favour of PKG_CHECK_MODULES"

This reverts commit a66cb8cf216536b4e5e35c98c3f114d1787131b1.

SDL 3 will have the recommended path forward, we don't need to nag in SDL 2.

sdl2.m4

@@ -10,7 +10,7 @@
 # * removed HP/UX 9 support.
 # * updated for newer autoconf.
 
-# serial 3
+# serial 2
 
 dnl AM_PATH_SDL2([MINIMUM-VERSION, [ACTION-IF-FOUND [, ACTION-IF-NOT-FOUND]]])
 dnl Test for SDL, and define SDL_CFLAGS and SDL_LIBS
@@ -19,7 +19,6 @@ AC_DEFUN([AM_PATH_SDL2],
 [dnl
 dnl Get the cflags and libraries from the sdl2-config script
 dnl
-AC_MSG_WARN([[$0 is deprecated, please use PKG_CHECK_MODULES([SDL], [sdl2 >= MINIMUM_VERSION]) instead]])
 AC_ARG_WITH(sdl-prefix,[  --with-sdl-prefix=PFX   Prefix where SDL is installed (optional)],
             sdl_prefix="$withval", sdl_prefix="")
 AC_ARG_WITH(sdl-exec-prefix,[  --with-sdl-exec-prefix=PFX Exec prefix where SDL is installed (optional)],