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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.