work with newer versions of automake

when using a newer automake, the autoreconf step fails with warnings:
"linking libtool libraries using a non-POSIX archiver requires 'AM_PROG_AR' in 'configure.ac' "

This happens for instance on ubuntu 13.10. 
Doing just that, adding 'AM_PROG_AR' to configure.ac fixes the problem.
Merge pull request #278 from polyester/master

work with newer versions of automake