This video from CodersVoice outlines the process pretty well.

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

• Project Settings > C/C++ Build > Settings, select Miscellaneous under GCC C++ Compiler, add the following to Other flags: -std=c++11
• Window > Preferences > C/C++ > Build > Settings, click the Discovery tab, select CDT GCC Built-in Compiler Settings MinGW [Shared], and to Command to get compiler specs add: -std=c++11

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.