Ok,
the Symbol is "wxMenuBar" in "ThirdPlugin.getDebugMenu" (_ZN11ThirdPlugin12getDebugMenuEP9wxMenuBar)
ldd told me, that it took libcodeblocks.so.0 from a f***ing old standard installation.
I removed /opt/gnome/lib/libcodeblocks.so.0 and changed the linker search directory from $(#cb)/lib to $(#cb.lib) and cb.lib=<c::b-install-dir>/lib
Now, "ldd -v ThirdPlugin.so" says that libcodeblocks.so.0 cannot be found.
What can I do about that? Must I add -lcodeblocks.so.0 to the linker options? Or reconfigure C::B because of pkg-config stuff?