The source and target attributes of <javac>
don't have any default values for historical reasons. Since the
underlying javac compiler's default depends on the JDK you use, you
may encounter build files that don't explicitly set those attributes
and that will no longer compile using a newer JDK. If you cannot
change the build file, Ant provides two properties that help you
setting default values for these attributes. If the attributes have
been set explicitly, the properties listed here will be ignored.
Since Ant 1.7
Provides a default value for <javac>'s and
<javadoc>'s source attribute.
Since Ant 1.7
Provides a default value for <javac>'s target
attribute.