ISTM that this would be better as a separate module or an optional submodule to posix.  The posix module is already 8720 lines.  I really don't want it to get bigger, especially when you realize how much #ifdef'ery is in there.  

Some other things I noticed:

You should use PyMem_Malloc instead of a raw malloc (same deal with free).  Methods that take no arguments should use METH_NOARGS and then there's no need to call PyArgs_ParseTuple (e.g., posix_cap_get_proc).  

There definitely shouldn't be any abort()s in there, even if #ifdef'ed out.

Is this 64-bit safe?  My manpage (gentoo) says this:  int  cap_set_flag(cap_t  cap_p,  cap_flag_t flag, int ncap, cap_value_t *caps, cap_flag_value_t value);

I see that you are using ints.  I don't know if that's correct on a 64-bit platform.  If not, you will need to modify the places that ints are used to take longs.