SCons: Implement minor fixes

[Repiteo]

Integrates a handful of adjustments to the SCons buildsystem:

• Ensure scope of Configure is properly closed. This is "handled" automatically by SCons, but it recommends this practice & will prevent accidental breakage if wanting to further configure down the road.
• Wrap post-build functions behind check for clean and help commands, so they only run when it's appropriate.
• Use SCons 4.0+ implementation of dumping environment information
• Ensure initial warning for lack of progress percentage is shown
• Ensure elapsed time centiseconds won't crop leading zeros

[Calinou]

Code looks good to me.