Is there a C++ library I can use for platform-agnostic "fork()" functionality?

We have an application that uses fork() and a few other process-related POSIX functions on Linux based operating systems. Instead of using fork() directly, is there some platform-agnostic API that I can use that provides this functionality?

We are using C++11. I have not seen functionality in STL for this, nor have I seen anything in Boost (which is available to us).

What is the most portable solution for this? I know I could create my own abstractions for CreateProcess() on Windows and fork() on Linux, but I find it hard to believe something like this hasn't already been done.