Because git is used in `python setup.py sdist`

    https://lab.nexedi.cn/nexedi/wendelin.core/blob/11d130d1/setup.py#L168

we need to make sure it is available at runtime (at time when sdist
happens), or else installation could fail.

Since zc.recipe.egg:develop now supports setting environment variables

    https://lab.nexedi.cn/nexedi/slapos.buildout/commit/93d35f5d
    https://lab.nexedi.cn/nexedi/slapos.buildout/commit/71078537 )

and we use the proper version of it in slapos stack:

    https://lab.nexedi.cn/nexedi/slapos/commit/d0807d90

we can just set environment with git being in PATH and fix it this way.

/cc @Tyagov, @kazuhiko, @jerome
/reviewed testbot(building ok), trustme

This reverts commit bf75e8a9.

also parallel build becomes possible again.

This checks if a certain parameter is the write one, normally it is required to know
if the value is not published but not yet ready.

Ruby2.1

To support GitLab we need current Ruby oldstable, which is Ruby 2.1.6.

Rationale: https://gitlab.com/gitlab-org/gitlab-ce/commit/757df0142f521380b92d28a721a7fd2bd8aa382f

/cc @kazuhiko

See merge request !5

Current Ruby stable is 2.2.2, but current GitLab does not work with it,
and requires current Ruby oldstable, which is 2.1.6:

    https://gitlab.com/gitlab-org/gitlab-ce/commit/757df0142f521380b92d28a721a7fd2bd8aa382f

/cc @kazuhiko

In the next patch we'll add another version of Ruby, and it will make
sense to share common configuration for Ruby compiling.

/cc @kazuhiko

Like with e.g. python or readline, or like with versioned programs e.g.
on Debian, put version into name without dash. So

    ruby-2.2 -> ruby2.2

/cc @kazuhiko

to avoid possible symbol lookup error, like "libreadline.so.6: undefined symbol: UP".