They are defined in python's platform dependent C source code, like for example [here](http://hg.python.org/cpython/file/712b4665955d/Python/dynload_win.c). They can be accessed like this:
```
[ f[0] for f in imp.get_suffixes() if f[2] == imp.C_EXTENSION ]
```
And in python 3.3 there is [_SUFFIXES](http://docs.python.org/3/library/importlib.html#module-importlib.machinery)