build: replace deprecated AM_CONFIG_HEADER with AC_CONFIG_HEADERS

automake-1.13 has now deprecated AM_CONFIG_HEADER, use the correct
AC_CONFIG_HEADERS instead.

Change-Id: I8adaec64cbad7f7318ff69091176c30b707cbb0b
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1117


Tested-by: jenkins
Reviewed-by: Mikko Viitamäki <mikko.viitamaki@gmail.com>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>

configure.ac

@@ -8,7 +8,7 @@ m4_include([config_subdir.m4])dnl
 AM_INIT_AUTOMAKE([-Wall -Wno-portability dist-bzip2 dist-zip])
 AM_MAINTAINER_MODE
 
-AM_CONFIG_HEADER([config.h])
+AC_CONFIG_HEADERS([config.h])
 AH_BOTTOM([
 #include <helper/system.h>
 #include <helper/types.h>