configure.ac: fixed for automake >= 1.12 which requires AM_PROG_AR

error message without the fix
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/usr/share/automake-1.14/am/ltlibrary.am: warning: 'libsancus-core.la': linking libtool libraries using a non-POSIX
/usr/share/automake-1.14/am/ltlibrary.am: archiver requires 'AM_PROG_AR' in 'configure.ac'
src/Makefile.am:3:   while processing Libtool library 'libsancus-core.la'
/usr/share/automake-1.14/am/ltlibrary.am: warning: 'libsancus-netlink.la': linking libtool libraries using a non-POSIX
/usr/share/automake-1.14/am/ltlibrary.am: archiver requires 'AM_PROG_AR' in 'configure.ac'
src/netlink/Makefile.am:7:   while processing Libtool library 'libsancus-netlink.la'
autoreconf: automake failed with exit status: 1
------------------------------------------------------------------------