bump to 2.59 baseline for autoconf (it -was- 5 yrs old, this year)

git-svn-id: https://svn.apache.org/repos/asf/apr/apr-util/branches/1.4.x@835661 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/configure.in b/configure.in
index c8d3a74..726f13f 100644
--- a/configure.in
+++ b/configure.in
@@ -2,7 +2,7 @@
 dnl Process this file with autoconf to produce a configure script
 dnl
 
-AC_PREREQ(2.50)
+AC_PREREQ(2.59)
 AC_INIT(export_vars.sh.in)
 
 AC_CONFIG_HEADER(include/private/apu_config.h)