For capn_write_fd() the write() function would be ideal to pass as
write_fd, but write() has type ssize_t (*)(int, const void *, size_t),
whereas write_fd expects ssize_t (*)(int, void *, size_t). Passing
write() directly with GCC 5.4 causes a warning
-Wincompatible-pointer-types (on by default).
Unless capn_segment is defined with __declspec(align(64)),
check_segment_alignment fails to compile in x86 mode, as (sizeof(struct
capn_segment)&7) -> (44 & 7) evaluates to 4
Always compiles in x64 mode, as (sizeof(struct capn_segment)&7) -> (80 &
7) evaluates to 0
