This video from CodersVoice outlines the process for Windows pretty well.

The per-project configuration details start at 6:39. In short:

1. Open Project Settings > C/C++ Build > Settings
2. Select Miscellaneous under GCC C++ Compiler
3. Add the following to Other flags: -std=c++11
4. Open Window > Preferences > C/C++ > Build > Settings
5. Click the Discovery tab
6. Select CDT GCC Built-in Compiler Settings MinGW [Shared]
7. Add: -std=c++11 to Command to get compiler specs

As far as I can tell, there's no easy way to make Eclipse CDT C++ projects use C++11 settings by default, so you'll have to do this for all projects that use C++11 features.