I think there is very little risk to trying this out, just make sure to create a backup of the file you modify. You can run a text editor as administrator, then use it to open and edit the file.

There are good reasons why WRI discourages people from modifying installation files: doing so can break things, sometimes in very subtle ways, and people might not even realize that the problems they see could be because they modified something several months ago and then forgotten about it. Then this creates an additional support burden for WRI. So they certainly don't want such modifications to become commonplace in the community.

However, in this specific case, I wouldn't worry. Just make a backup of the file and do keep in mind that you made this particular modification.

Thanks, Szabolcs. You're right -- "VS140COMNTOOLS" is correct, per the environment variable that I've just learned to access (Control Panel / System / System / AdvSystemSettings / AdvancedTab / EnvironmentVariables then scroll down in the box).

Now I need to unprotect VisualStudioCompiler.m so that I can edit it. Years ago, Wolfram Tech Support said "Check with us first before editing *.m files" so I'll wait 'til Monday to do so. I'll then post to Stack Exchange, as you requested (yes, that was indeed the thread -- I now see that you had participated in it).

The procedure is described here:

• http://mathematica.stackexchange.com/a/41691/12

This is probably what you referred to, but it's better to actually post the link.

VS120COMNTOOLS is an environment variable, so you need to check what similarly named environment variables exist on your system.

VS 2013 has version number 12.0 and VS 2015 has version number 14.0. So I suppose it must be VS140COMNTOOLS.

Microsoft seems to be supersititious because they skipped version 13.0.