Silence automake warning about missing AM_PROG_AR

/usr/share/automake-1.14/am/ltlibrary.am: warning: 'libm210.la': linking libtool libraries using a non-POSIX
/usr/share/automake-1.14/am/ltlibrary.am: archiver requires 'AM_PROG_AR' in 'configure.ac'

By ignoring the warning we lose almost nothing:

An excerpt from the automake manual:
> extra-portability
>
>     extra portability issues related to obscure tools. One example of such
>     a tool is the Microsoft lib archiver.
>