...
<build>
<pluginManagement>
...
<plugins>
<plugin>
<groupId>org.eclipse.xtend</groupId>
<artifactId>xtend-maven-plugin</artifactId>
<version>2.4.2</version>
<executions>
<execution>
<goals>
<goal>compile</goal>
</goals>
</execution>
</executions>
<configuration>
<outputDirectory>${basedir}/xtend-gen</outputDirectory>
</configuration>
<dependencies>
<dependency>
<groupId>org.eclipse.xtend</groupId>
<artifactId>org.eclipse.xtend.lib</artifactId>
<version>2.4.2</version>
<type>pom</type>
</dependency>
</dependencies>
</plugin>
</plugins>
</pluginManagement>
</build>
...
<pluginRepositories>
<pluginRepository>
<id>sonatype_releases</id>
<url>https://oss.sonatype.org/content/repositories/releases/</url>
<releases>
<enabled>false</enabled>
</releases>
<snapshots>
<enabled>true</enabled>
</snapshots>
</pluginRepository>
<pluginRepositories>
...