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