The --warn / .WARNINGS feature

[Paul Smith]

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.