Added comments concerning __DARWIN_UNIX03 vs __APPLE__
diff --git a/support.h b/support.h
index cba12d7..e1e7203 100644
--- a/support.h
+++ b/support.h
@@ -4,6 +4,9 @@
#ifdef __APPLE__
// Darwin (Mac OS) only: disk IOCTLs are different, and there is no lseek64
+// This used to use __DARWIN_UNIX03 rather than __APPLE__, but __APPLE__
+// is more general. If the code fails to work on older versions of OS X/
+// Darwin, this may need to be changed back (and in various .cc files).
#include <sys/disk.h>
#define lseek64 lseek
#else