Modules were introduced in Java 9 and are not supported by the default
icedtea compiler, so this feature is disabled by default.
* guix/build-system/ant.scm (ant-build): Add use-java-modules?
parameter.
* guix/build/ant-build-system.scm (default-build.xml)
(configure): Use it.
Change-Id: I3b99238e4cd262332fa5c818be1af5477c7374fd