This is for cases where you need a value representing the OS at
runtime. If you're doing something which should compile differently
on different OSes, then please use version(Windows),
version(linux), etc.
This is intended for cases where you need to deal with endianness at
runtime. If you're doing something which should compile differently
depending on whether you're compiling on a big endian or little
endian machine, then please use version(BigEndian) and
version(LittleEndian).