This definitely sounds like a linker problem. Does the documentation tell you which library the QRCodeDetector class lives? Are all of your rebuilt libraries compiled using the same compiler, compilation settings, bitness, and debug vs. release version?

For the Microsoft Visual C++ linker, the order of libraries is not relevant; the linker figures out from the exported methods in all of the libraries what needs to be dragged in. MinGW probably uses the GNU linker, where libraries have to be in dependency order. For Visual C++, there is the Dependency Walker tool, which shows the hierarchy of inter-library dependencies. Is there a similar tool in the MinGW / GNU world? I don't know - almost all of my C++ development is on Windows with Microsoft toolchains.