Use the PACKAGE_BUGREPORT macro that is created by Autoconf for
referring to the bug reporting address rather than hardcoding it
everywhere.  This makes it easier to change the address and it reduces
translation work.
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://www.postgresql.org/message-id/flat/8d389c5f-7fb5-8e48-9a4a-68cec44786fa%402ndquadrant.com