configure.ac: Add AM_PROG_AR for automake-1.12

configure.ac: Add AM_PROG_AR for automake-1.12

This is what automake told me to do:

  $ ./autogen.sh
  ...
  automake: warnings are treated as errors
  /usr/share/automake-1.12/am/ltlibrary.am: warning: 'libcomedi.la': linking libtool libraries using a non-POSIX
  /usr/share/automake-1.12/am/ltlibrary.am: archiver requires 'AM_PROG_AR' in 'configure.ac'
  lib/Makefile.am:2:   while processing Libtool library 'libcomedi.la'
  ...
  autoreconf-2.69: automake failed with exit status: 1

AM_PROG_AR is new in automake-1.11.2.  For further details, see:

  http://lists.gnu.org/archive/html/bug-automake/2012-05/msg00009.html