Deduplicate `llgramarea` from RXD.

[1uc]

There two copies of llgramarea: https://github.com/search?q=repo%3Aneuronsimulator%2Fnrn%20llgramarea&type=code

This causes issues with Cython, because it's dlsym'ed, but also available via Cython. The latest version of Cython uses name manging when generting C++ sources, i.e. we can't use dlsym easily. When turning off namemanging, we get issues with escaping the quotes properly on MinGW, i.e. we need to define a variable CYTHON_EXTERN_C='extern "C"' at some point while passing down the variable through Python code and MinGW shell we end up with the equivalent of:

#define CYTHON_EXTERN_C extern C

Note that none of the code related to CYTHON_EXTERN_C is under our control, it's all handled for us by Cython.