Remove acconfig.h altogether by removing XML_BYTE_ORDER. We can simply

Remove acconfig.h altogether by removing XML_BYTE_ORDER. We can simply
use the BYTEORDER constant that ac_c_bigendian_cross.m4 generates.

* accconfig.h: removed; obsolete.

* lib/winconfig.h:
  - set the BYTEORDER constant, rather than XML_BYTE_ORDER, and leave
    a note about what we're assuming.

* lib/xmltok.c, lib/xmltok_ns.c:
  - change all XML_BYTE_ORDER references to BYTEORDER, and
    update the constants from 12, 21 to 1234, 4321 respectively.