This commit addresses the following warnings reported on gcc 5.2.1. In
the future, this will help reduce the "noise" and help catch warnings
revealing a serious problem.

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/path/to/libzmq/tests/test_stream_exceeds_buffer.cpp: In function ‘int main()’:
/path/to/libzmq/tests/test_stream_exceeds_buffer.cpp:19:9: warning: variable ‘rc’ set but not used [-Wunused-but-set-variable]
     int rc = setsockopt(server_sock, SOL_SOCKET, SO_REUSEADDR, &enable, sizeof(enable));
         ^
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