Upgrade to Autoconf version 2.53. Replaced many custom macro

calls with new or now-built-in versions.  Make sure that all
calls to AC_DEFINE have a third argument, for possible use of
autoheader in the future.

src/template/aix

@@ -6,4 +6,5 @@ else # not GCC
     aix3.2.5 | aix4.1*)
       CFLAGS='-qmaxmem=16384 -qsrcmsg' ;;
   esac
+  CCC=xlC
 fi # not GCC