build: restore configure formatting, add final message for warnings

[silverwind]

This is a follow-up on #483. It fixes the readability concern of #595 by restoring the options print to its previous pretty-printed form while adding a final print in case if warnings happened.

I also took the liberty to change the color of warnings from light red to light yellow, as I think that color is more appropriate and red should be reserved for errors only.

[rvagg]

fine by me, can you give us a screenshot or fixed-format paste into here?

[silverwind]

Here you go, had to reduce the font size a bit to fit everything on this tiny screen.

[rvagg]

lgtm but I'd like signoff from @bnoordhuis too cause I know he did some refactoring of #483 in #585 and may also have suggestions for wording (I don't really like "Careful! Warnings happened" but can't think of a good alternative).

[silverwind]

I'm open to any suggestions regardind the print and its color. Also noticed I could refactor the final check by removing the msg variable, if that style is prefered.

[bnoordhuis]

I believe it's slightly more pythonic to replace the global with:

defwarn(msg):
# ...warn.warned=Truewarn.warned=False

[silverwind]

Good call, I'll fix that.

[silverwind]

@bnoordhuis updated with your suggestions.

[silverwind]

@bnoordhuis wait, I'll push another update as I noticed this when no warnings occured:

AttributeError: 'function' object has no attribute 'warned'

[bnoordhuis]

You need to define warn.warned, else you get an AttributeError below when warn() hasn't been called.

On the topic of the commit log, can you make the first line fit in <= 50 characters? Thanks!

[silverwind]

Whoops, looks like I've broken this PR. Won't let me re-open it while the branch is still active. Guess I need to open a new one, sorry.