Add check for sys/uio.h to configure script.

sys/uio.h contains the XSI vector I/O data structure.

configure.in

@@ -246,6 +246,9 @@ stdlib.h string.h sys/socket.h sys/time.h time.h unistd.h limits.h)
 # Check if we have ifaddrs.h header file.
 AC_CHECK_HEADERS(ifaddrs.h, [AC_DEFINE(ZMQ_HAVE_IFADDRS, 1, [Have ifaddrs.h header.])])
 
+# Check if we have sys/uio.h header file.
+AC_CHECK_HEADERS(sys/uio.h, [AC_DEFINE(ZMQ_HAVE_UIO, 1, [Have uio.h header.])])
+
 # Force not to use eventfd
 AC_ARG_ENABLE([eventfd], [AS_HELP_STRING([--disable-eventfd], [disable eventfd [default=no]])],
     [zmq_disable_eventfd=yes], [zmq_disable_eventfd=no])