QPID-5102: C++ Broker Windows build with PROTON needs new _EXTERN statements.

Note that this patch supplies only _EXTERNs. It does not change the build for finding PROTON nor does it correct the QPID-5100 amqp/amqpc library export issues.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1518337 13f79535-47bb-0310-9956-ffa450edef68