This will largely depend on the used platform and other constraints. As a general rule, a good (C++) programmer is (or should be) able to learn a platform-specific API in a very short time. For C++, it's much more important to understand the different tool chains (e.g. a Windows programmer should also know the GCC tool chain) and differences in compilers. Programmer should also understand limitations and platform-dependend behaviour of the language.
As for libraries, C++ programmers absolutely need to know STL and Boost. No discussion.
