The problem was discovered as part of issue #296.

AM_PROG_AR was introduced in Automake 1.11.2. The m4_ifdef() check around
its use was there to make it possible to regenerate the `configure`
script with `autoreconf` also on systems with older Automake
versions. However, `ar` is required and not running AM_PROG_AR will lead
to build failures.

Starting with Automake 1.12, AM_PROG_AR is "required" when building
libraries with Automake. libsrtp does not use Automake, but has a
handwritten Makefile.in template instead. That template has a AR variable
that `configure` is supposed to set.

The logic for doing so comes from AM_PROG_AR, which does a AC_SUBST() on
the variable to set it. If AM_PROG_AR is not available/used, AR will not
be set and the build will fail with rather obscure message.

There's only two solutions to this; either do what this patch does, which
is to require Automake 1.11.2 or newer on systems that are to regenerate
the `configure` script, or write a "else" part to the m4_ifdef() check
and roll your own AR check. The latter is probably non-trivial given
various systems, cross compilers, etc.