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
