Add support for C++ linkage to cluster membership API.
Without this change, C++ programs will not link to the library.

BKrev: 40d53e4bzcKNiPnfX523_065NBw4gg


git-svn-id: http://svn.fedorahosted.org/svn/corosync/trunk@20 fd59a12c-fef9-0310-b244-a6a79926bd2f