Bug#792390: gradle: improve bootstrapping by using groovyc

[Emmanuel Bourg]

It looks like Fedora gave up with Gradle [1], I'm not sure the upgrades
are significantly easier with this approach.

[1] 
https://src.fedoraproject.org/rpms/gradle/c/4a126e8e3380eda7ceda22b18a9d9633bcd8cda8?branch=rawhide