AC_PATH_PROG needs absolute path only.
AC_CHECK_PROG can use a program in path.

https://bugs.gentoo.org/721030
--- a/configure.ac
+++ b/configure.ac
@@ -162,7 +162,7 @@ esac
 #
 AC_CONFIG_FILES([make/rules make/includes])
 
-AC_PATH_PROG(AR, ar)
+AC_CHECK_PROG(AR, ar)
 ARFLAGS="cruv"
 AC_SUBST(AR)
 AC_SUBST(ARFLAGS)