Backport Win32 LevelDB env from C++0x to C++

Since the gitian mingw compiler doesn't support C++0x yet.

src/leveldb/util/env_boost.cc

@@ -401,7 +401,7 @@ class PosixEnv : public Env {
 
       boost::interprocess::file_lock fl(fname.c_str());
       BoostFileLock * my_lock = new BoostFileLock();
-      my_lock->fl_ = std::move(fl);
+      fl.swap(my_lock->fl_);
       if (!my_lock->fl_.try_lock()) {
           return Status::IOError("database already in use: could not acquire exclusive lock");
       }