This uses AM_PROG_AR to discover ar, which is the recommended way to do so. Among other advantages, it honors the AR environment variable (as set from the outside). The macro has been around since automake 1.11.2 (Dec 2011).
This commit also removes code that discovers ranlib and strip. ranlib has been obsolete for decades (ar does its task now automatically), and anyway LT_INIT takes care of discovering it. The code we used to set STRIP was last mentioned in the automake 1.5 manual. Since automake 1.6 (Mar 2002), strip is discovered automatically when necessary (look for the private macro AM_PROG_INSTALL_STRIP in the automake manual).
Alternative to #1086.