Support for C++11 Libraries

[sathibault]

The -std=c++11 may conflict with other platform libraries and cannot always be added to a project build_flags. There should be some way for the library.json to be able to specify this. I'm not sure just having an extra flags attribute in the library.json is sufficient since the base flags may already have some -std option.

[ivankravets]

Duplicate of #289