make distclean: Don't delete $SAGE_ROOT/local

[mkoeppe]

#21501, #21479 turn SAGE_LOCAL into a configurable install hierarchy. Consequently, $SAGE_ROOT/local should not be deleted by make distclean.

[jdemeyer]

I would argue that $SAGE_LOCAL should not be deleted by make distclean but that $SAGE_ROOT/local should be deleted by make distclean.

[mkoeppe]

That's certainly a defendable position.

What if the user explicitly used --prefix=$SAGE_ROOT/local or to another subdirectory of $SAGE_ROOT?

[embray]

I would lean toward make distclean removes any $SAGE_LOCAL that's a subdirectory of $SAGE_ROOT. That's the point of make distclean. There's not much use for a custom --prefix outside installing Sage outside of $SAGE_ROOT.

[mkoeppe]

Moving some tickets to 9.2. This is not a promise that I will be working on them.

[mkoeppe]

I think it's time to clean up the various -clean targets in the top-level Makefile so that they match autotools behavior

[mkoeppe]

Setting new milestone based on a cursory review of ticket status, priority, and last modification date.