2012-01-13.

In a recent (started last August) gentoo system, some packages 
have had trouble in merging, e.g. gdm, several with errors about
an image library not recognising a particular file's format.

Traditionally I've just emerged things, done removals if needed
to avoid clashes that prevent desired upgrades, and finally built 
a fully new system every few years (until KDE3 finished, and KDE4
was still unusable, so I had to keep an old system).

But here are some finesses for making packages happy about /all/ 
dependencies being present, and removing cruft (pulled in ebuilds)
that no longer are needed.

Note that "@world" denotes the list (stored in /var/lib/portage/world )
of all packages that have been /asked for/ (emerge XXXX); the total 
installed set (listed under /var/db/pkg/) should be these plus all 
dependencies.

# check that all the possible (build as well as runtime) dependencies
# of installed packages are present
emerge --update --newuse --deep --with-bdeps=y @world

# check/remove any packages no longer required by the @world list
emerge --depclean -pv

(when run properly, without -pv, this then runs commands to 
 correct problems by any removed gcc,
  gcc-config x86_64-pc-linux-gnu-4.4.5
  fix_libtool_files.sh 4.4.5
)

# and check further for any breakages (from removal of old libraries)
(?  rv -v -r /var/cache/revdep-rebuild/*  ?)
revdep-rebuild -pv

# to ensure X11 drivers match the latest server
emerge `qlist -I -C x11-drivers/`

# and possibly rebuild everything, to use new gcc etc, 
emerge -e @world