Bug#866929: "Unable to find javadoc command:" on openjdk-9

[Chris West]

Package: src:maven-javadoc-plugin
Version: 2.10.4-1
Priority: normal

On OpenJDK 9, the plugin fails to find the javadoc command, and prints:

[ERROR] Failed to execute goal
  org.apache.maven.plugins:maven-javadoc-plugin:2.10.4:jar (default-cli)
  on project [...]:
MavenReportException: Error while generating Javadoc:
  Unable to find javadoc command:
    The environment variable JAVA_HOME is not correctly set. -> [Help 1]

Please ensure that it can find the javadoc command, at least during
Debian package builds.

This is causing around 160 packages to fail to build against openjdk-9:
https://paste.debian.net/974403/
A good example is `java-string-similarity`.


I couldn't find an upstream issue for this, which suggests it may be an
issue with openjdk-9 itself... or that everyone else is just setting
JAVA_HOME. Setting JAVA_HOME does fix it, but it may be easier to fix
the plugin than all the various build scripts?

I can't provide a non-package testcase, as maven is currently broken due
to #866924.