test/acinclude.m4 (AM_PATH_SDL2): apply recent sdl2.m4 PKG_CONFIG patch

test/configure

@@ -3966,7 +3966,7 @@ fi
 
   if test "x$sdl_pc" = xyes ; then
     no_sdl=""
-    SDL2_CONFIG="pkg-config sdl2"
+    SDL2_CONFIG="$PKG_CONFIG sdl2"
   else
     as_save_PATH="$PATH"
     if test "x$prefix" != xNONE && test "$cross_compiling" != yes; then