autotools: bump minimum needed autoconf version to 2.65.

older versions seem to output a broken configure script.

configure.ac

@@ -1,5 +1,5 @@
 dnl Process this file with autoconf to produce a configure script.
-AC_PREREQ([2.63])
+AC_PREREQ([2.65])
 AC_INIT
 AC_CONFIG_SRCDIR([src/SDL.c])
 AC_CONFIG_HEADERS([include/SDL_config.h])