Fix endian detection in configure.ac

On debian-mips, neither __LITTLE_ENDIAN__ nor __BIG_ENDIAN__ are defined
anywhere, so PLIST_BYTE_ORDER defaults to PLIST_LITTLE_ENDIAN when it should
really be PLIST_BIG_ENDIAN on this architecture.

This fixes issue #13.
Drop manual endian.h header check

AC_C_BIGENDIAN should do its job already.