The same specialization of ProcessorCache:

   template class ProcessorCache<std::size_t, ProcessorRcPtr>;

is explicitly instantiated in two different translation units:

    src/OpenColorIO/Processor.cpp
    src/OpenColorIO/Config.cpp

which violates the C++ standard (an explicit instantiation definition
shall appear at most once in a program).

Since you are compiling with C++11 (vs. C++98), you can change the line
in Config.cpp to

   extern template class ProcessorCache<std::size_t, ProcessorRcPtr>;

and it should be fine (in theory, I haven’t run a scratch build).