gnu: Add and use gettext-minimal.

gnu: Add and use gettext-minimal.

* gnu/packages/gettext.scm (gnu-gettext): Rename to...
(gettext-minimal): ... this.  Adjust synopsis and description.
(gnu-gettext): Inherit from it.
(po4a): Use 'gettext-minimal' instead of 'gnu-gettext'.
* gnu/packages/acl.scm: Likewise.
* gnu/packages/admin.scm: Likewise.
* gnu/packages/apl.scm: Likewise.
* gnu/packages/attr.scm: Likewise.
* gnu/packages/audio.scm: Likewise.
* gnu/packages/base.scm: Likewise.
* gnu/packages/cdrom.scm: Likewise.
* gnu/packages/commencement.scm: Likewise.
* gnu/packages/crypto.scm: Likewise.
* gnu/packages/databases.scm: Likewise.
* gnu/packages/disk.scm: Likewise.
* gnu/packages/documentation.scm: Likewise.
* gnu/packages/education.scm: Likewise.
* gnu/packages/engineering.scm: Likewise.
* gnu/packages/enlightenment.scm: Likewise.
* gnu/packages/fcitx.scm: Likewise.
* gnu/packages/fontutils.scm: Likewise.
* gnu/packages/freedesktop.scm: Likewise.
* gnu/packages/games.scm: Likewise.
* gnu/packages/gkrellm.scm: Likewise.
* gnu/packages/glib.scm: Likewise.
* gnu/packages/gnome.scm: Likewise.
* gnu/packages/grub.scm: Likewise.
* gnu/packages/gtk.scm: Likewise.
* gnu/packages/guile.scm: Likewise.
* gnu/packages/ibus.scm: Likewise.
* gnu/packages/irc.scm: Likewise.
* gnu/packages/iso-codes.scm: Likewise.
* gnu/packages/kde-frameworks.scm: Likewise.
* gnu/packages/kodi.scm: Likewise.
* gnu/packages/linux.scm: Likewise.
* gnu/packages/man.scm: Likewise.
* gnu/packages/maths.scm: Likewise.
* gnu/packages/mono.scm: Likewise.
* gnu/packages/mp3.scm: Likewise.
* gnu/packages/music.scm: Likewise.
* gnu/packages/nano.scm: Likewise.
* gnu/packages/networking.scm: Likewise.
* gnu/packages/package-management.scm: Likewise.
* gnu/packages/pdf.scm: Likewise.
* gnu/packages/sawfish.scm: Likewise.
* gnu/packages/statistics.scm: Likewise.
* gnu/packages/terminals.scm: Likewise.
* gnu/packages/version-control.scm: Likewise.
* gnu/packages/vpn.scm: Likewise.
* gnu/packages/w3m.scm: Likewise.
* gnu/packages/webkit.scm: Likewise.
* gnu/packages/wicd.scm: Likewise.
* gnu/packages/wine.scm: Likewise.
* gnu/packages/xdisorg.scm: Likewise.
* gnu/packages/xorg.scm: Likewise.