Not all toolchains on UNIX-style operating systems predefine
"unix". For example, it's missing on NetBSD, OpenBSD/gcc, AIX,
HP-UX. There is no single macro defined everywhere, but checking
both "__unix__" and "__unix" should cover everything except macOS,
which is already checked for using "__APPLE__".
Note that case sensitivity should default to off on macOS and
cygwin, so the check there is different.
