You guys are the experts. I can only comment on what I see. IMHO: the file _sysconfigdata.py is more accurate with nothing in it.

I am clearly confused by whatever process this is. If you believe it is more accurate to have the BLD variable 'inaccurate' in this file - and that we just need to "believe" that it has the correct value during the build (which, starting with version 2.7.13 seems to be the case, as I am able to build in a different directory - and could not do this with version 2.7.10 (first time I tried using separate directories).

In short, I have no expert opinion on this. My naive view is that both variables being accurate in the file is more accurate. In any case, the patch is needed for a working _sysconfigdata.py and integration with something such as "pip build ...". Only after I learned about 'pip' did I ever run into this, i.e., only people who are trying to install something extra have issues here. I have been 'patching' _sysconfigdata.py manually so that things work normally.

I hope my feedback helps you move forward on this.