Unit test workaround for AIX.

asio/src/tests/unit/unit_test.hpp

diff --git a/asio/src/tests/unit/unit_test.hpp b/asio/src/tests/unit/unit_test.hpp
index 5d7e10b..ddbe9b4 100644
--- a/asio/src/tests/unit/unit_test.hpp
+++ b/asio/src/tests/unit/unit_test.hpp
@@ -112,4 +112,13 @@
 {
 }
 
+#if defined(__GNUC__) && defined(_AIX)
+
+// AIX needs this symbol defined in asio, even if it doesn't do anything.
+int test_main(int, char**)
+{
+}
+
+#endif // defined(__GNUC__) && defined(_AIX)
+
 #endif // UNIT_TEST_HPP