Multiple warnings in Automake >1.12, AM_PROG_AR in configure.ac

From autoreconf -fiv:

    autoreconf: running: automake --add-missing --copy --force-missing
    /usr/local/automake/share/automake-1.12/am/ltlibrary.am: warning: 'libnanox-c.la': linking libtool libraries using a non-POSIX
    /usr/local/automake/share/automake-1.12/am/ltlibrary.am: archiver requires 'AM_PROG_AR' in 'configure.ac'
    src/apis/c/debug/Makefile.am:24:   while processing Libtool library 'libnanox-c.la'
    /usr/local/automake/share/automake-1.12/am/ltlibrary.am: warning: 'libnanox-c.la': linking libtool libraries using a non-POSIX
    /usr/local/automake/share/automake-1.12/am/ltlibrary.am: archiver requires 'AM_PROG_AR' in 'configure.ac'
    src/apis/c/instrumentation-debug/Makefile.am:24:   while processing Libtool library 'libnanox-c.la'
    ...

New versions of Automake require AM_PROG_AR macro but Automake 1.11 doesn't recognize it. These warnings can be hidden by adding -Wno-extra-portability in AM_INIT_AUTOMAKE options; or include AM_PROG_AR by adding m4_ifdef([AM_PROG_AR], [AM_PROG_AR]) before AC_PROG_LIBTOOL.