I pushed my changes to implement the --warn / .WARNINGS feature. I hope people find them useful. As of now there are only 3 possible warnings, but I will be looking through other warnings that might be useful to control.
There is one major caveat to this: currently it's not possible to set .WARNINGS as a target-specific variable and override warning settings for just that target (and prerequisites). Making this work ended up being tricky as currently "special variables" are not checked at all during target-specific handling, and the way warnings work we are not re-interpreting the value of that variable every time we want to check how to handle a warning. I will think about whether there's a good way to deal with this but for now, you can't control warnings on a per-target basis.
