[WIP] MacOS build

Almost there, but I get a malloc error on importing the Python wrapper:

```
python(31952) malloc: *** error for object 0x7fff7a62a570: pointer being freed was not allocated
*** set a breakpoint in malloc_error_break to debug
/bin/sh: line 1: 31952 Abort trap: 6           /Users/vene/envs/pystruct/bin/sphinx-build -q -b html -c /Users/vene/code/opengm/build/src/interfaces/python/_build -d /Users/vene/code/opengm/build/src/interfaces/python/opengm/_doctrees /Users/vene/code/opengm/src/interfaces/python/docsrc/source /Users/vene/code/opengm/build/src/interfaces/python/opengm/html
```

Thanks!