Minor fixes found while trying to compile it using clang on Mac OS X

include/rocksdb/env.h

@@ -18,8 +18,13 @@
 #include <memory>
 #include <vector>
 #include <stdint.h>
+#include <sys/types.h>
 #include "rocksdb/status.h"
 
+#ifdef __APPLE__
+typedef off_t off64_t;
+#endif
+
 namespace leveldb {
 
 class FileLock;
@@ -28,7 +33,7 @@ class RandomAccessFile;
 class SequentialFile;
 class Slice;
 class WritableFile;
-class Options;
+struct Options;
 
 using std::unique_ptr;
 using std::shared_ptr;