gem: explicitely set --no-format-executable

Gem, unlike Debian, sets it by default to ON (it seems). This has the
very unfortunate effect that one cannot guess how the bin scripts
are going to be called ...
release 1.9.7.rc4
fix mixup in ruby version detection

We now use ruby_executable everywhere the full path to the ruby
executable is needed instead of accessing raw values.
define lsb_release for opensuse
another fix for ruby version detection
Merge github.com:doudou/autoproj
arch: add the --needed option on install to avoid reinstalling up-to-date packages
gem: make sure that default options are actually the default options

Distributions like arch and opensuse override the gem defaults in
their /etc/gemrc, breaking the expected behaviour under autoproj.

Make the default options explicit, and add a variable to allow
the user to configure it if he so chooses.
don't refer to manjaro explicitly, it should be resolvable as arch
osdeps: display which OS has been detected to the user
need to define Autobuild.progress and .progress_done in bootstrap