Partial workaround problems with javadoc plugin and Java 11

* force java home without the jre subdirectory
* handle different source version and java version

pom.xml

@@ -226,6 +226,12 @@
 					<groupId>org.apache.maven.plugins</groupId>
 					<artifactId>maven-javadoc-plugin</artifactId>
 					<version>3.2.0</version>
+					<configuration>
+						<!-- Workaround for: Unable to find javadoc command: The environment variable JAVA_HOME is not correctly set. -->
+						<javadocExecutable>${java.home}/bin/javadoc</javadocExecutable>
+						<!-- Workaround for: The code being documented uses modules but the packages defined in https://docs.oracle.com/javase/8/docs/api/ are in the unnamed module. -->
+						<source>${project.source.version}</source>
+					</configuration>
 				</plugin>
 				<plugin>
 					<groupId>org.apache.maven.plugins</groupId>