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The standard is clearly defining that only <poll.h> needs to be included.

http://pubs.opengroup.org/onlinepubs/000095399/basedefs/poll.h.html

--- a/lib/m4/usual.m4	2013-08-09 15:16:56.716600440 +0200
+++ b/lib/m4/usual.m4	2013-08-09 15:17:07.488877797 +0200
@@ -165,7 +165,7 @@
 dnl  AC_USUAL_HEADER_CHECK:  Basic headers
 dnl
 AC_DEFUN([AC_USUAL_HEADER_CHECK], [
-AC_CHECK_HEADERS([sys/socket.h poll.h sys/poll.h sys/un.h])
+AC_CHECK_HEADERS([sys/socket.h poll.h sys/un.h])
 AC_CHECK_HEADERS([arpa/inet.h netinet/in.h netinet/tcp.h])
 AC_CHECK_HEADERS([sys/param.h sys/uio.h pwd.h grp.h])
 AC_CHECK_HEADERS([sys/wait.h sys/mman.h syslog.h netdb.h dlfcn.h])
--- a/lib/usual/socket.h	2013-08-09 15:10:58.444331854 +0200
+++ b/lib/usual/socket.h	2013-08-09 15:14:24.957124489 +0200
@@ -22,7 +22,6 @@
  * - win32: <winsock2.h>
  * - win32: <ws2tcpip.h>
  * - <sys/socket.h>
- * - <sys/poll.h>
  * - <sys/un.h>
  * - <netinet/in.h>
  * - <netinet/tcp.h>
@@ -50,9 +49,6 @@
 #ifdef HAVE_POLL_H
 #include <poll.h>
 #endif
-#ifdef HAVE_SYS_POLL_H
-#include <sys/poll.h>
-#endif
 #ifdef HAVE_SYS_UIO_H
 #include <sys/uio.h>
 #endif