Use correct directory for database files.
Bug: https://bugs.gentoo.org/783195

--- a/Makefile.in
+++ b/Makefile.in
@@ -40,7 +40,7 @@ MANDEST = @mandir@
 # The root of the directory tree for read-only
 datarootdir = @datarootdir@
 # Pathname of directory to install database file
-ARPDIR ?= @prefix@/arpwatch
+ARPDIR ?= @datarootdir@/arpwatch
 
 # VPATH
 srcdir = @srcdir@