Improvements:
- use of system installed python in the build-all.py script for Mac OS X
- reverted to the use of swig 3.0.12 for the time being becaue swig version 4.0 causes error in the build process with Mac OS X
- use of the to_pystring() was minimized

…, some whitespaces corrected

* 'v0.6.0' of github.com:atibaut/IfcOpenShell:
  Only pull when on a branch
  -pthread on unix
  fix po::value
  Threads in python app
  Cleanup concurrently created breps and triangulations
  Write progress for threaded conversion
  Some fixes to threading work
  Implement multi_threaded implementation in Iterator
  Last commit for multithreading, Airsquire implementation
  Sander boer's change for multithreading 1. Added multithread for create element 2. Added support function in IfcConvert.cpp for multithreading at bottom
  .clang-format settings indent=2 for me, temporarily. style=allman width=80
  builds nicely, but chokes immediately on serializer- first element, needs debugging
  Linearizing the use of IfcGeomIterator by integrating it into IfcConvert in full, atm it does not build.
  merge with ifcopenshell
  Too lazy to comment

Example: python build-all.py --use_system_python