Improve handling of GCC_DEP variable in Makefile

[embray]

This is a followup to #25188 that I thought was worth doing while fixing that issue. However, since it's not strictly needed for the fix I'll make it a separate ticket.

Depends on #25188

CC: @vbraun

Component: build

Author: Erik Bray

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/25198

[embray]

Dependencies: #25188

[sagetrac-git]

Changed commit from 14b6f4d to b33c210

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. Last 10 new commits:

2965003	add vim modeline for this file
f5f68da	Add a package that by all rights should be included in this list
15c7b88	move this list into a TOOLCHAIN_DEPS variable we can use to inspect this list elsewhere
1954e5e	Add -k/--keep-existing to sage-spkg
f1411c5	From -clean targets in the Makefile, just remove the package's
9976091	Use sage-spkg --keep-existing when installing/re-installing packages in
22a1f34	Revert to using sage-spkg-uninstall in -clean targets, but add a
d71c88f	Merge branch 'u/embray/ticket-25857' into u/embray/build/ticket-25188
f062068	fix accidentally removed blank line needed to make debugging output look better
b33c210	Reduce some unnecessary text duplication in the Makefile.

[embray]

comment:4

I believe this issue can reasonably be addressed for Sage 8.4.

[dimpase]

comment:5

Toolchain packages are unconditionally built - it's also not addressed on #24919.
Anyhow this ought to be dealt with a follow-up to #24919.

[dimpase]

Reviewer: Dima Pasechnik