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.