configure.ac: Bump libx264 requirement to a more recent version (my version had x264_nal_encode, but X264_ME_UMH was ...

Original commit message from CVS:
* configure.ac:
Bump libx264 requirement to a more recent version (my version had
x264_nal_encode, but X264_ME_UMH was missing from the header file,
among other things)
* ext/x264/x264enc.c:
Use "_stdint.h" instead of <stdint.h>
* ext/x264/x264enc.h:
* ext/mad/gstid3demuxbin.c:
HAVE_CONFIG_G => HAVE_CONFIG_H