ext2fs: don't use O_DIRECT if not available

O_DIRECT is not defined on OSX.  Since direct IO is only a new
optimization and not needed for correct functionality, disable
it if O_DIRECT is unavailable.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
diff --git a/lib/ext2fs/unix_io.c b/lib/ext2fs/unix_io.c
index 6e480b7..dddaf6f 100644
--- a/lib/ext2fs/unix_io.c
+++ b/lib/ext2fs/unix_io.c
@@ -474,8 +474,10 @@
 	open_flags = (flags & IO_FLAG_RW) ? O_RDWR : O_RDONLY;
 	if (flags & IO_FLAG_EXCLUSIVE)
 		open_flags |= O_EXCL;
+#ifdef O_DIRECT
 	if (flags & IO_FLAG_DIRECT_IO)
 		open_flags |= O_DIRECT;
+#endif
 	data->flags = flags;
 
 #ifdef HAVE_OPEN64