revert replace AM_PATH_SDL2 usage with corresponding PKG_CHECK_MODULES.

Reference issue: https://github.com/libsdl-org/SDL/issues/6494 .

test/configure.ac

@@ -101,7 +101,10 @@ AC_SUBST(ISOS2)
 
 dnl Check for SDL
 SDL_VERSION=2.0.18
-PKG_CHECK_MODULES([SDL], [sdl2 >= $SDL_VERSION],, AC_MSG_ERROR([*** SDL version $SDL_VERSION not found!]))
+AM_PATH_SDL2($SDL_VERSION,
+            :,
+	    AC_MSG_ERROR([*** SDL version $SDL_VERSION not found!])
+)
 CFLAGS="$CFLAGS $SDL_CFLAGS"
 LIBS="$LIBS -lSDL2_test $SDL_LIBS"