If ./configure detects an existing GCC installed in Sage (SAGE_INSTALL_GCC=exists) then it should keep gcc selected among the packages to be installed as part of the distribution. Otherwise it basically deselects gcc which is the wrong thing to do (with any package).

Depends on #25857

CC: @embray

Component: build

Author: Erik Bray

Branch/Commit: 5d0a981

Reviewer: Volker Braun, Julian Rüth, Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/25188