Fix -Wundef warning

Was thrown on 32 bit, should be #ifdef instead of #if because the attribute
is only defined on 64 bit machines.
Silence yet another warning

I have no clue why neither gcc nor clang throw a -Wsign-compare for me
there, but do for some other people, so... *sigh*
remove old static versionLabel text
move © years out of translatable text in aboutDlg
adapt translations to previous change
update quassel.pot