Allows use of this library in macOS - #132

I'm curious how does MacOS handle python2 vs python3? The extended include path doesn't seem to include a version number.

I've just started playing with this (very useful!) library on OSX using Python 3. It all seems to work fine and I didn't need to do anything unusual to get Python.h to be available. My cmake reports this:

Python_VERSION:3.7.7
Python_NumPy_VERSION:1.18.3
Python_LIBRARIES:/usr/local/Cellar/python/3.7.7/Frameworks/Python.framework/Versions/3.7/lib/libpython3.7m.dylib
Python_INCLUDE_DIRS:/usr/local/Cellar/python/3.7.7/Frameworks/Python.framework/Versions/3.7/include/python3.7m
Python_NumPy_INCLUDE_DIRS:/usr/local/lib/python3.7/site-packages/numpy/core/include

and Python.h is available in the Python include dir.

Sorry, I may have got confused. I can quite happily use this library, i.e. the header file and the various python libraries and header files in my own project. This works fine. However, I can't build this project on my Mac either. I get the same error. I think it might be to do with how the cmake file is set up. I am investigating...

Using the provided Makefile and Python 3 I'm not having any issues on Mac (Catalina, 10.15.4). I'm not sure if this conditional change is ideal.