Simplify build/install process for bki and description files. There's

Simplify build/install process for bki and description files.  There's
no reason for them to be copied into src/backend rather than being
installed straight from the catalog subdirectory.  This also avoids
some peculiar behavior (bugs?) present in at least gmake 3.78.1: it
won't always update the bki files in backend/ even when the ones in
backend/catalog/ are newer.