Fix the build on OS X (10.9.3)

In particular OS X doesn't have BLKGETSIZE64 or O_DIRECT.

Signed-off-by: David Scott <dave.scott@citrix.com>
Fix the build back on Linux again

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #23 from djs55/osx

Fix the build on OS X (10.9.3)