Create a subproject with lib template and put all the classes and functions you want to use in plugins (and remove them from the main project). Then tell the main project to link with the library (for example using LIBS+=-L pathtosubproject -lnameoflib). Do the same with all the plugins (I mean link them against the newly created library). Remember that this library has to be found by the dynamic loader when you start your main app (you can use LD_LIBRARY_PATH env variable to do that).
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