aviau authored Jul 15, 2016 and Guillaume Roguez committed Jul 20, 2016

This adds the support for the DISABLE_CONTRIB_DOWNLOADS environment
variable. It if it set to TRUE, the contrib system will not download
tarballs from the internet.

It can be set from the bootstrap script with the --disable-downloads
option.

This is useful for distro packaging because distribution developpers
have to make sure they have full control of what is included in the
package. In Debian, some tarballs are included such as pjsip but the
rest should not be downloaded. A small mistake such as a missging build
dependency could cause a tarball download. This new options will help
pervent such issues.

Tuleap: #847
Change-Id: I5f579774c53def751111f9366750670fcd75d893