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)

 reporter_name="Harold Grovesteen"

 reporter_url="http://sourceware.org/ml/crossgcc/2009-11/msg00052.html"

 reporter_comment=""