configure: Fix warnings on CentOS and bump the minimum autoconf version

* configure.ac: Bump the minimum version to 2.60 as needed by
  AC_USE_SYSTEM_EXTENSIONS.  AC 2.60 is nearly 10 years old, note.  Add
  AC_PROG_RANLIB, for when --disable-shared is used.

  There are other warnings on, e.g., CentOS 6.7 with 2.63, but they
  don't go away if the suggestion to add AC_SYSTEM_EXTENSIONS is
  followed. This warning doesn't occur on Fedora with AC 2.69.

  Note: autoconf (and other auto*) should only be needed on developer
  machines building direct from git.  Other systems should be using the
  'make dist' tarballs, with a ready-made build system, that does not
  need auto* intalled.
diff --git a/configure.ac b/configure.ac
index c684414..0fc9c09 100755
--- a/configure.ac
+++ b/configure.ac
@@ -5,7 +5,7 @@
 ##  Copyright (c) 1996, 97, 98, 99, 2000 Kunihiro Ishiguro <kunihiro@zebra.org>
 ##  Portions Copyright (c) 2003 Paul Jakma <paul@dishone.st>
 ##
-AC_PREREQ(2.53)
+AC_PREREQ(2.60)
 
 AC_INIT(Quagga, 0.99.25, [https://bugzilla.quagga.net])
 CONFIG_ARGS="$*"
@@ -70,6 +70,7 @@
 AC_PROG_CC
 AC_PROG_CPP
 AM_PROG_CC_C_O
+AC_PROG_RANLIB
 AC_PROG_EGREP
 
 dnl autoconf 2.59 appears not to support AC_PROG_SED