This patch to build.xml avoids these warnings from ant (by dropping the
'whenempty' attribute entirely):
jar:
[jar] JARs are never empty, they contain at least a manifest file
[jar] Building jar: /usr/src/pgsql/src/interfaces/jdbc/jars/postgresql.jar
[jar] JARs are never empty, they contain at least a manifest file
[jar] Building jar: /usr/src/pgsql/src/interfaces/jdbc/jars/postgresql-examples.jar
-O