Building jar targeting multiple Java versions, including 9

[Oliver Gondža]

Thank you all for your suggestions. I managed to get the project to 
build with following maven setup:

```
   <!--
         When using compiler from java 8 and older, ignore module files 
altogether.
         Otherwise, use 2 phase compilation to build
           - all classes for target version and
           - module-info.java with 9+ source and target level
      -->
   <build>
     <plugins>
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-compiler-plugin</artifactId>
         <version>3.5.1</version>
         <configuration>
           <source>1.6</source>
           <target>1.6</target>
         </configuration>
         <executions>
           <execution>
             <id>default-compile</id>
             <configuration>
               <excludes>
                 <exclude>**/module-info.java</exclude>
               </excludes>
             </configuration>
           </execution>
         </executions>
       </plugin>
     </plugins>
   </build>

   <profiles>
     <profile>
       <id>jigsaw</id>
       <activation>
         <jdk>[1.9,)</jdk>
       </activation>
       <build>
         <plugins>
           <plugin>
             <groupId>org.apache.maven.plugins</groupId>
             <artifactId>maven-compiler-plugin</artifactId>
             <version>3.5.1</version>
             <configuration>
               <source>1.9</source>
               <target>1.9</target>
             </configuration>
             <executions>
               <execution>
                 <id>module-infos</id>
                 <phase>compile</phase>
                 <goals>
                   <goal>compile</goal>
                 </goals>
                 <configuration>
                   <includes>
                     <include>**/module-info.java</include>
                   </includes>
                 </configuration>
               </execution>
             </executions>
           </plugin>
         </plugins>
       </build>
     </profile>
   </profiles>
```

It does compile with older javac versions as a bonus. Given this is 
nothing else than using `-source 9 -target 9` for module-info.java if 
present, I dare to say maven-compiler-plugin can be adapted to figure 
this out on its own.

-- 
oliver