3 issues

[Philippe91]

const static uint8_t lenbits = FLAGLEN;

should be:

const static uint8_t lenbits = (7 - FLAGLEN);

uint64_t remaining_length = unit.length >> DeltaHeadUnit::lenbits;
write_varint(buffer, remaining_length);

should be:

if (uint64_t remaining_length = unit.length >> DeltaHeadUnit::lenbits)
	write_varint(buffer, remaining_length);

#ifdef _WIN32
    output_fd = open(cvalue, O_RDWR | O_TRUNC | O_CREAT);
#else

should be (else eof added!):

#ifdef _WIN32
    output_fd = open(cvalue, O_RDWR | O_TRUNC | O_CREAT | O_BINARY);
#else