bug with use_dll, k_order_perturbation

The following tests fail:

|     * k_order_perturbation/fs2000k2_use_dll.mod
|     * k_order_perturbation/fs2000k_1_use_dll.mod
|     * k_order_perturbation/fs2000k3_use_dll.mod

with the error:dynare:k_order_perturbation: Caught KordDynare exception: ../../../sources/k_order_perturbation/dynamic_dll.cc:70: Error when loading ./fs2000k2_use_dll_dynamic.mexa64 (can't locate the 'Dynamic' symbol)

We first found this on Matlab R2016a, though it may have existed earlier. This bug exists on OS X, Linux and Windows with MinGW (thanks @JohannesPfeifer ). This does not exist on Windows with MSVC.

Can possibly fix with extern "C" or by creating C++ mex files. Should see in which version of Matlab this problem started and why.