When a .tarball-version file containing the version string is present,
it is taken for granted that the source tarball is present.  This
allows to lift the requirement on a Git checkout to use the source
tarball.  This makes it easy and cheap to sync this tarball and
accompanying .tarball-version file across different Jenkins agents to
distribute the packaging jobs, bring the total build time from about 1
h 40 to 40 minutes.

* Makefile (TARBALL_VERSION): New variable.  When defined, do not do
not invoke Git to derive the version string, simply use it.
[RELEASE_TARBALL_FILENAME]: Do not compute prerequisites when
TARBALL_VERSION is defined; assume the tarball is present.
* .gitignore: Ignore .tarball-version.
* Jenkinsfile: (Generate release tarball): Generate a .tarball-version
file, and stash it along the release tarball.  Explicit this should
run on a 'guix' agent.
(Build packages): Forward to any 'linux-builder' agent, clean the
workspace, unstash and extract the tarball and build.

GitLab: jami-packaging#98
Change-Id: I13088b75c9ccd5166662a84b7c5f8d4e8b7b65da

* Jenkinsfile [parameters]: New directive.
(Build packages): Skip building custom Qt or ARM packages depending on
the parameter values.

GitLab: jami-packaging#98
Change-Id: Ic68af873483ac92319061baf5f14ef7323a27a26

The phony tarball-producing targets will now always clear the
tarballs.manifest file before generating a new release tarball,
ensuring it doesn't contain stale information.

A new 'portable-release-tarball' target is also added to offer an easy
to use target that was previously implemented ad-hoc in the CI.  Move
this logic here as a portable release tarball is useful for packagers
wanting to test locally with the various supported platforms.  The
Jenkinsfile now makes use of it.

* Makefile (daemon/contrib/native/Makefile): New target.
(tarballs.manifest): Add the above target as a prerequisite and remove
invocation to bootstrap script.  Invoke make with --no-print-directory
to prevent entering/leaving directory messages from appearing in the
output (it's supposed to be on when using --silent, but when the make
invocation is nested, it doesn't seem to be the case).
(release-tarball): Also clear the tarballs.manifest when invoked.
(purge-release-tarballs, portable-release-tarball): New targets.
* guix/minimal-manifest.scm: New file.
* Jenkinsfile ('Generate release tarball): Use it to spawn the
container used to build the release tarball.  Also expose /usr/bin/env
in the container.  Use the new 'portable-release-tarball' target to
simplify things.  Fix the shebang line; it needs to be the first line
of the script.

GitLab: jami-packaging#98
Change-Id: I3b55e25933108ae9930bc9fcc867f9fa51d796f2

The pipeline code is modernized to use the declarative pipeline DSL of
Jenkins, and the GNU/Linux package builds now all share the same
source release archive, which is built from cached contribs tarballs
when available.

GitLab: jami-packaging#55
Change-Id: Id7f37966dd80c36cfa0ebdd461c0ab90505dcd37