TLDR:

  To enable Greenstone to support Google Sigin, run the following to
  install the needed jar files:
  
  ./01-DOWNLOAD-JAR-ASSEMBLIES.sh
  ./02-LOCATE-JAR-DEPENDENCIES.sh
  ./03-INSTALL-JAR-DEPENDENCIES.sh

  The first sript downloads the relevant jar assemblies from the main
  maven repository.  As Greenstone3 is an 'ant' build infrastruture,
  we then need to pick out from the downloaded files the jar files  
  we need to add into the Greenstone3 to support compilation.  This
  is what the 2nd and 3rd scripts do.


For geneeral info about using Google API Java Client see:

  https://developers.google.com/api-client-library/java/google-api-java-client/setup
  https://developers.google.com/api-client-library/java/google-api-java-client/download

The 'google-api-client' jar is built on top of 'google-oauth-client' and 'google-http-client'

The latest versions of these jars can be downloaded via:

  https://search.maven.org/artifact/com.google.api-client/google-api-client-assembly

    https://search.maven.org/artifact/com.google.oauth-client/google-oauth-client-assembly
    https://search.maven.org/artifact/com.google.http-client/google-http-client-assembly

At the time of writing, the versions added in were:
    google-api-client:   v1.32.1
    google-oauth-client: v1.31.5
    google-http-client:  v1.39.2