As of glibc 2.19, the following macros are defined in `endian.h`:
  htobe16(), htole16(), be16toh(), le16toh(), htobe32(), htole32(),
  be32toh(), le32toh(), htobe64(), htole64(), be64toh(), le64toh()

These macros are also defined in FreeBSD, where they are located
in the `<sys/endian.h>` file.

These defines are not used anywhere, and are non-standard, and
don't appear on FreeBSD.
(variables starting with `__` should generally not be used).

Sparse is only used for checking the Linux kernel, we do not use
it in this project.