Fix autoconf warnings


git-svn-id: svn://svn.code.sf.net/p/libjpeg-turbo/code/trunk@1122 632fc199-4ca6-4c93-a231-07263d6284db
Older versions of automake don't support AM_PROG_AR, so only execute that macro if it exists.


git-svn-id: svn://svn.code.sf.net/p/libjpeg-turbo/code/trunk@1123 632fc199-4ca6-4c93-a231-07263d6284db