Setup.hs: Better way of handling man page building.

Previously we tried to remove make-pandoc-man-pages from the list
of packages to be haddocked, installed, copied, etc.

It works better to set 'Buildable: False' on make-pandoc-man-pages,
then have the buildHook temporarily set Buildable to True.  This
allows make-pandoc-man-pages to be built (and used in generating
the man pages), but not installed.