You need to link against the c++ standard library. This normally happens automatically if your compile sources phase contains any c++ files, but if the c++ is all contained within a static library then you need to explicitly ask for it to be linked to the final executable.
Check the subproject's build settings. Look for the C++ Standard Library build option. If it's set to libc++ (llvm standard library), then add -lc++
to your application's additional linker flags. If it's set to libstdc++ (gnu library), then add -lstdc++
to your application's additional linker flags.