Fix for #128 and adds support for C++11 feature `sleep_for`

This commit adds a patch to the gcc build that fixes the problem on darwin,
that the C++11 `sleep_for` methods are not available. The fix is a patch
that was introduced by macports to fix the same problem.

https://trac.macports.org/ticket/36364

This patch should become irrelevant either with 4.8 or 4.7.3.