gettext: Check for existence of stdenv.ccCross.
gettext: Check for existence of stdenv.ccCross.

Should only affect cross-builds, however I didn't test whether
cross-building gettext actually works. But if it's broken now, it was
broken before as well and this at least unbreaks the non-cross-built
nixpkgs tarball job :-)

Signed-off-by: aszlig <ee1aa092358634f9c53f01b5a783726c9e21b35a@redmoonstudios.org>

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