Fix building termcap.db when make obj is run beforehand from a clean tree by
using make variables for the filenames, which helps resolve pathing
appropriately when running cap_mkdb

X-MFC with: r275687
Pointyhat to: me


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275692 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f