configure: replace deprecated AM_CONFIG_HEADER with AC_CONFIG_HEADERS

AM_CONFIG_HEADER has been removed in the just-released automake 1.13:
https://lists.gnu.org/archive/html/automake/2012-12/msg00038.html

https://bugzilla.gnome.org/show_bug.cgi?id=690881
Fuck libav. Long live ffmpeg!