Introduces a new program called gradle for managing files stored in
the home directory by the [Gradle Build Tool](https://gradle.org).
Gradle uses the $HOME/.gradle folder for all it's configuration.
Features of the new program module are:
- Automatically setting programs.java.enable = true to make a Java
installation available for running Gradle.
- Specifying an alternate Gradle home directory
- Setting of abitrary values for gradle.properties stored inside the
Gradle home directory.
- Defining init scripts that will be linked into the init.d inside
the Gradle home directory.
Co-authored-by: Olli Helenius <liff@iki.fi>
Co-authored-by: Robert Helgesson <robert@rycee.net>
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