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

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.

(cherry picked from commit f9f4731245eb9f83d0795acac24183c6cf709288)