Fix errors while building with clang (against libc++) under linux

I got several building errors with clang (against libc++) under linux.
And I made this small patch which works for me.

```
export CXX="clang++ -std=c++11 -stdlib=c++"
export LDFLAGS="-lc++ -lc++abi"
make -j8
```

Of course, you should build gflags against libc++ instead of libstdc++.