Im not sure I follow - other than updating gcc/g++ in a completely separate location (C:\msys64) then I have followed the PYMC install instructions exactly as given, and as mentioned above, this has worked previously. It would also appear that v5.3 g++ is bundled with PYMC, and used by default?
Given the compile errors, and also bearing in mind the comments in my previous link: Getting a Working Pymc3 Windows 10 Installation using Anaconda - An Installation Guide
suggesting updating to a more recent g++, I started to look here first for the source of the error, hence the comments about msys paths in my original post.