I investigated that issue a bit further...
MakePlug() is defined in IPlug_include_in_plug_src.h inside some Preprocessor #ifdefs.
I recognized that although IPlug_include_in_plug_src.h is part of the ...-aax project the code inside #elif defined AAX_API directive is greyed out while the code inside #if defined VST_API is not. It seems as if the wrong API is recognized here which would IMHO explain the unresolved symbol.
I verified the project properties and the C++ Preprocessor definitions evaluate correctly to AAX_API.
I don't know when and how the false API recognition occured
Maybe anyone has an idea why this could happen or how to solve this?