Add DARWIN_USE_64BIT_INODE on OSX to match stat.h

On MacOSX when DARWIN_USE_64BIT_INODE is specified ino_t is ulong and the
stat structure members are switched around.

In particular osxfuse compiles with DARWIN_USE_64BIT_INODE (see https://github.com/osxfuse/fuse/blob/fuse-2.7/osxfuse.pc.in) leading to ABI incompatibility between the library and D code when using the stat structure.

A bit more detailed description on why to use DARWIN_USE_64BIT_INODE:
__DARWIN_64_BIT_INO_T is used in sys/stat.h to check if we should use the STAT64 struct.
sys/cdefs sets this to 1 if _DARWIN_USE_64_BIT_INODE is set.