configure: Use modern way to set AR

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 sets RANLIB and STRIP. ranlib is
obsolete since the 1980s (ar does its task now automatically). The code
we used to set STRIP was last mentioned in the automake 1.5 manual.
Since automake 1.6 (Mar 2020), strip is discovered automatically when
necessary (look for the *private* macro AM_PROG_INSTALL_STRIP in the
automake manual).

configure.ac

@@ -34,15 +34,12 @@ m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])
 
 PKG_PROG_PKG_CONFIG
 
-AC_PATH_TOOL(AR, ar)
-AC_PATH_TOOL(RANLIB, ranlib)
-AC_PATH_TOOL(STRIP, strip)
-
 AC_PROG_CC
 if test x"$ac_cv_prog_cc_c89" = x"no"; then
   AC_MSG_ERROR([c89 compiler support required])
 fi
 AM_PROG_AS
+AM_PROG_AR
 
 build_windows=no