Provide own definitions for sigaction(2) etc.

The definitions from posix/signal.pxd were broken before Cython 0.24:
cython/cython@ced17f8

Fixes:

    afl.pyx:130:36: Cannot assign type 'sighandler_t' to 'void *(*)(int)'

Author: jwilk

Diff for: afl.pyx

@@ -48,7 +48,21 @@ from libc.stddef cimport size_t
 from libc.stdint cimport uint32_t
 from libc.stdlib cimport getenv
 from libc.string cimport strlen
-from posix.signal cimport sigaction, sigaction_t, sigemptyset
+
+cdef extern from 'signal.h':
+    # We could use definitions from posix/signal.pxd,
+    # but these were broken before Cython 0.24.
+    ctypedef struct siginfo_t:
+        pass
+    ctypedef struct sigset_t:
+        pass
+    cdef struct sigaction_t 'sigaction':
+        void sa_handler(int)
+        void sa_sigaction(int, siginfo_t *, void *)
+        sigset_t sa_mask
+        int sa_flags
+    int sigaction(int, const sigaction_t *, sigaction_t *)
+    int sigemptyset(sigset_t *)
 
 cdef extern from 'sys/shm.h':
     unsigned char *shmat(int shmid, void *shmaddr, int shmflg)