build-sys: add AM_PROG_AR

build-sys: add AM_PROG_AR

This silences these warnings when running bootstrap.sh:

/usr/share/automake-1.13/am/ltlibrary.am: warning: 'module-augment-properties.la': linking libtool libraries using a non-POSIX
/usr/share/automake-1.13/am/ltlibrary.am: archiver requires 'AM_PROG_AR' in 'configure.ac'
augment/Makefile.am:1:   while processing Libtool library 'module-augment-properties.la'
/usr/share/automake-1.13/am/ltlibrary.am: warning: 'module-dir-watch.la': linking libtool libraries using a non-POSIX
/usr/share/automake-1.13/am/ltlibrary.am: archiver requires 'AM_PROG_AR' in 'configure.ac'
augment/Makefile.am:1:   while processing Libtool library 'module-dir-watch.la'
augment/Makefile.am: installing './depcomp'
/usr/share/automake-1.13/am/ltlibrary.am: warning: 'module-combine-sink.la': linking libtool libraries using a non-POSIX
/usr/share/automake-1.13/am/ltlibrary.am: archiver requires 'AM_PROG_AR' in 'configure.ac'
combine/Makefile.am:1:   while processing Libtool library 'module-combine-sink.la'
/usr/share/automake-1.13/am/ltlibrary.am: warning: 'module-murphy-ivi.la': linking libtool libraries using a non-POSIX
/usr/share/automake-1.13/am/ltlibrary.am: archiver requires 'AM_PROG_AR' in 'configure.ac'
murphy/Makefile.am:14:   while processing Libtool library 'module-murphy-ivi.la'

Change-Id: Ie802041272952e581d900b5edef91d682192c0f8