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>