configure: fix --mandir

--mandir points to the base dir of the man pages, so
we have to append our man section below given dir.
(transplanted from a8a4d5c9773893fa6978e5f9e1445d1e8185c24a)

# Compute IA-64-specific values

CT_DoArchTupleValues() {

    # The architecture part of the tuple:

    CT_TARGET_ARCH="${CT_ARCH}"

}