There are at least two packages ("graphics/cairo" and "x11/xlsclients")
which require this. And we definitely want to avoid the situation where
both the based and "pkgsrc" version get used at the same time.
Bump all the packages which use the "x11/libxcb" package except the two
which already require version 1.6. For those package remove the manually
set minimum required version.
Change discussed with Thomas Klausner in private e-mail.
