--mandir points to the base dir of the man pages, so
we have to append our man section below given dir.
(transplanted from a8a4d5c9773893fa6978e5f9e1445d1e8185c24a)
1 This is NOT the official version of dialog. This version has been
2 significantly modified from the original. It is for use by the Linux
3 kernel configuration script. Please do not bother Savio Lam with