Move config.h include line to appropriate place before include lo.h.
Allow LO_BIGENDIAN to be overridden by compile-time macros during universal build.

Previously the LO_BIGENDIAN macro ended up assigned the empty string
("") by AC_C_BIGENDIAN during universal build.  This patch gives it a
default special value of "2", which is then used in lo_endian.h to
re-assign the value according to the __BIG_ENDIAN__ or
__LITTLE_ENDIAN__ if they exist.