update makefile.unix to provide 'make extras' building instructions. note: input ordering depends on WEBP_SWAP_16BIT_CSP for rgb565 and rgb4444 Change-Id: I6f22d32189d9ba2619146a9714cedabfe28e2ad0
meant to contain additional utility functions useful in processing webp input/output. Change-Id: I014ae6b917d62e826aa23a3bbe99aac4462a97c2