lzwenc stores a function pointer to either put_bits or put_bits_le;
however, after the recent change, the function pointer's prototype
would depend on BitBuf. BitBuf is defined in put_bits.h, whose
definition depends on whether BITSTREAM_WRITER_LE is #defined or not.
For safety, we set a boolean flag for little/big endian instead,
which also allows the definition to be inlined.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>