bug#24861: [PATCH] Fix annoying "Parsing...done" message in c++-mode

[Eli Zaretskii <eliz@gnu.org>]

> From: Hong Xu <hong@topbug.net>
> Cc: 24861@debbugs.gnu.org
> Date: Tue, 15 Nov 2016 16:03:19 -0800
> 
> Can you still consider this patch?

I'm okay with accepting this for the master branch, but the patch
needs some more work to fix the following issues:

 . The first line of each doc string should be a complete sentence.
 . The doc string of cpp-progress-message should mention
   cpp-message-min-time-interval.
 . The defcustom you are adding should have a :version tag.
 . The calculation in cpp-progress-message should be fixed to
   calculate the time difference between the current time and the time
   of the previous progress message, and compare that with the value
   of cpp-message-min-time-interval.  The old code just looked at the
   2nd member of the list returned by current-time, but that is no
   longer TRT when you need to compare the time difference, because
   that member can go back to zero.  You need to use time-subtract.
 . Last, but not least: please include ChangeLog-style commit log
   message for the changes.

Thanks.