bug#32868: Check if make supports nested variables is done twice when using silent rules

[Zack Weinberg]

> i'm inclined to bring this back as the way to opt-in to silent-rules
> by default.

And I’m still absolutely opposed to making it even *possible* to have silent 
rules be on by default. In fact I’d like to see —enable-silent-rules removed, 
with make V=0 the *only* way to switch to silent mode.