configure.ac: bump AC_PREREQ to 2.60
All other xorg modules require at least 2.60 (released in 2006), so we
may as well increase it to match. It's also doubtful anyone tests the
build with 2.59 (from 2003), so it may not even work anyway.
diff --git a/configure.ac b/configure.ac
index 51face7..8f481a0 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1,6 +1,6 @@
dnl Process this file with autoconf to create configure.
-AC_PREREQ([2.59])
+AC_PREREQ([2.60])
dnl Versioning - scrape the version from configs/default
m4_define([mesa_version],