scons: show sources and targets when building, and colorize output.

scons: show sources and targets when building, and colorize output.

I like the brevity of Ali's recent change, but the ambiguity of
sometimes showing the source and sometimes the target is a little
confusing.  This patch makes scons typically list all sources and
all targets for each action, with the common path prefix factored
out for brevity.  It's a little more verbose now but also more
informative.

Somehow Ali talked me into adding colors too, which is a whole
'nother story.