add new AC_PROG_AR helper

add new AC_PROG_AR helper

This mirrors the existing RANLIB and related toolchain macros that
autoconf already exports.  Some projects assume `ar` is available
which isn't always safe, so provide a macro that probes the full
toolchain settings.

This also makes it easier to use AC_REQUIRE with the macro instead
of duplicating the AC_CHECK_TOOL call in projects.

  * lib/autoconf/programs.m4 (AC_PROG_AR): New macro.
  * doc/autoconf.texi: Document it.
  * tests/local.at (_AT_CHECK_ENV): Allow $AR output variable.