Bugfix: AM_PROG_AR is not recognized by older automake, so only use it w...

...hen defined

Building on a variant of a FreeBSD 7.x machine, `autoreconf -fiv` fails:

> autoreconf-2.68: Entering directory \`.'
> autoreconf-2.68: configure.ac: not using Gettext
> autoreconf-2.68: running: aclocal --force
> configure.ac:23: warning: macro \`AM_PROG_AR' not found in library
> autoreconf-2.68: configure.ac: tracing
> autoreconf-2.68: running: libtoolize --copy --force
> libtoolize: putting auxiliary files in \`.'.
> libtoolize: copying file \`./ltmain.sh'
> libtoolize: Consider adding \`AC_CONFIG_MACRO_DIR([m4])' to configure.ac and
> libtoolize: rerunning libtoolize, to keep the correct libtool macros in-tree.
> libtoolize: Consider adding \`-I m4' to ACLOCAL_AMFLAGS in Makefile.am.
> configure.ac:23: warning: macro \`AM_PROG_AR' not found in library
> autoreconf-2.68: running: /usr/local/bin/autoconf-2.68 --force
> configure.ac:23: error: possibly undefined macro: AM_PROG_AR
>       If this token and others are legitimate, please use m4_pattern_allow.
>       See the Autoconf documentation.
> autoreconf-2.68: /usr/local/bin/autoconf-2.68 failed with exit status: 1