configure.ac: replace AM_CONFIG_HEADER with AC_CONFIG_HEADERS

automake 1.13 errors if AM_CONFIG_HEADER is used in configure script.
automake 1.13 no longer autoupdates AM_CONFIG_HEADER to
AC_CONFIG_HEADERS, thing which automake has been doing since automake
version 1.7

Given that our first automake supported version is automake 1.7,
simply replacing AM_CONFIG_HEADER usage with AC_CONFIG_HEADERS seems
enough to yet support same automake versions.

Dave Reisner reported issue with 1.13 and provided patch.

http://curl.haxx.se/mail/lib-2012-12/0246.html

configure.ac

@@ -35,7 +35,7 @@ This configure script may be copied, distributed and modified under the
 terms of the curl license; see COPYING for more details])
 
 AC_CONFIG_SRCDIR([lib/curl_urldata.h])
-AM_CONFIG_HEADER(lib/curl_config.h include/curl/curlbuild.h)
+AC_CONFIG_HEADERS(lib/curl_config.h include/curl/curlbuild.h)
 AC_CONFIG_MACRO_DIR([m4])
 AM_MAINTAINER_MODE