Semisync plugins to link statically into MariaDB

Please consider the attached patch that makes both semisync plugins linking statically into MariaDB. The patch contains only changes to plugins' code and build infrastructure. It doesn't have changes for tests because I don't know how would you prefer to change them, i.e. do you want them to make working only with built-in plugins (the same way as they now work only with dynamically loaded plugins) or you want to make them smart and working with both ways of building these plugins.