To download the upstream sources directly:
- direct a navigator to "http://libre.adacore.com/libre/download"
- select "Free Software or Academic Development"
- click on the "Build Your Download Package" button
- select any platform and year $(year)
- deploy the "GNAT Ada $(year)" item
- deploy the "Sources" subitem.
- click on the "gprbuild-gpl-$(year)-src.tar.gz" hyperlink (not the checkbox)

The upstream archive is repackaged with
# mk-origtargz gprbuild-gpl-$(year)-src.tar.gz \
  --package=gprbuild --version=$(year)dfsg \
  --exclude-file=$(exclude1) --exclude-file=$(excluded2) ..
Please refer to the copyright file in this directory to know which
files are excluded and why.

To compare with previous version,
  tar xf gprbuild_$(year).tar.gz
  diff -urN $(previous_version_directory) gprbuild-$(year)-src
You may want to ignore the sources in the gnat/ subdirectory,
as they are unused anyway.

 -- Nicolas Boulenguez <nicolas@debian.org>, Mon,  7 Sep 2015 22:07:26 +0200
