Output sync completed (?)

[Paul Smith]

Hi all.  I've recently pushed changes to solve the last open issues that
I'm aware of with the --output-sync feature:

      * If command line printing is not suppressed ("@" is not used) the
        command line is attached to the output.
      * Extraneous enter/leave lines are not printed any longer.
      * I renamed the options to -Oline, -Otarget (not changed) and
        -Orecurse.
      * I took another stab at documentation; please check the "Parallel
        Output" node and see if it is more understandable now.  Some
        picky details of -Otarget I didn't spell out but hopefully it's
        more clear.

Please test the heck out of this with the different modes in your most
difficult build environments, and let me know if you see any anomalies.
I'm also interested to know if the enter/leave stuff is really correct
now; it is in my testing but it sort of happened more by accident than
real planning.

We are right on the cusp of a release candidate for the next GNU make
version.  If you have the ability to build and test from Git feel free
to start your testing early.