Using AsciidoctorJ Diagram

The new plugins have built-in support for asciidoctorj-diagram. Simply add the following to the project or task extension and diagramming will be available. If it is not set the asciidoctorj-diagram JAR will not be added to the classpath.

asciidoctorj {
    modules {
       diagram.use() (1)
       diagram.version '1.5.16' (2)
    }
}

1	Enables diagram usage with whatever the default version of `asciidoctorj-diagram` is.
2	Enables usages, but set a specific version rather than the default suggested by the plugin.

With this enhancement, there is also no longer a need to add requires 'asciidoctor-diagram'.

If you are using OpenJDK 9/10 on macOS you might find an error such as below

Caused by: java.lang.UnsatisfiedLinkError: /path/to/openjdk10/lib/libfontmanager.dylib: dlopen(/path/to/openjdk10/lib/libfontmanager.dylib, 1): Library not loaded: /Users/jenkins/workspace/openjdk10_build_x86-64_macos/openjdk/installedfreetype/lib/libfreetype.6.dylib
  Referenced from: /path/to/openjdk10/lib/libfontmanager.dylib
  Reason: image not found

The solution is to install freetype via HomeBrew or MacPorts. You might also need to do something (ridiculous) such as

$ sudo mkdir -p /Users/jenkins/workspace/openjdk10_build_x86-64_macos/openjdk/installedfreetype
$ sudo ln -s /opt/local/lib /Users/jenkins/workspace/openjdk10_build_x86-64_macos/openjdk/installedfreetype/lib (1)

1	`opt/local/lib` is the location for MacPorts. Change it accordingly for HomeBrew.