Mingw32 build fixes

Here's another fix for the win32_inno build, hopefully the last one from me for now. ;-)

Its main purpose is to get rid of the stuff we currently install in the system32 folder. There's no real reason we need to install anything there, it is just copied over from the program directory, and the latter is all that's needed to run Purr. In fact it raises an issue, since we apparently don't properly clean this up for some reason, giving a seemingly never-ending series of prompts about overwriting files in system32 (without an option to simply accept all) when doing an upgrade or reinstall. This happens even if the existing installation, as recommended, is removed during the installation, by answering "Yes" to the corresponding prompt of the installer. I've seen this also on my students' computers, and it is a major source of annoyance and confusion when upgrading Purr.

The second commit silences a warning about the {pf} folder designation, which is deprecated in Inno Setup 6 and has been replaced with {commonpf}.

Both fixes have been in the win64_inno build from the get-go, this MR simply ports them over to the mingw32 build.

Tested on Windows 10 with Inno Setup 6, both with the light package (make light) and the normal one (just make), works fine for me, and resolves both issues.

NB: The next thing we should probably think about is what additional changes will be needed to permit per-user installations. This should be perfectly possible now with the changes in this MR, but probably needs some editing of the installation directories and the registry key designations. But that's something for another day.