The file twisted/internet/cfsupport/cfsupport.c (which is only compiled on OS X) is generated by an old version of Pyrex and contains non-standard C. Re-generating this file with Pyrex 0.9.4 solves the problem.
spkg: http://boxen.math.washington.edu/home/jdemeyer/spkg/twisted-9.0.p3.spkg
Component: packages: standard
Author: Jeroen Demeyer
Reviewer: Volker Braun
Merged: sage-5.0.beta6
Issue created by migration from https://trac.sagemath.org/ticket/12425