Hi guix,

This addresses things like

     ;; TODO(core-updates): make this unconditional
     `(,@(if (%current-target-system)
             `(("bash-minimal" ,bash-minimal)) ; for glib-or-gtk-wrap
             '())

It also introduces a mozilla-build-system, which is like gnu-build-system
except it overrides --target and --host to what mozilla software expects
and uses it for all Mozilla software I could identify that needs this.

I also removed a 'canonical-package' in isc-dhcp's input, because as I
understand it, ‘canonical packages’ aren't supposed to appear in the
references, except for gcc:lib and glibc perhaps?  (Not 100% sure,
but noone suggested using canonical-package when I told people to add
bash-minimal to package inputs.)

I also added a #:sh argument to wrap-qt-program to address a TODO(core-updates)
in guix/lint.scm and for consistency with wrap-program, but didn't test it
because of the many rebuilds.

I noted #:tests? wasn't respected in some packages I modified, which I fixed in
a few places.  I also removed trailing #t for some packages.

avahi and atk appear to (cross-)compile.  (I reverted
‘build/minetest-build-system: Move png-file? to (guix build utils).’ first to avoid
some rebuilds though.)

Greetings,
Maxime