Enables shelling out from a ruby process started with rbenv to a ruby
process with a different RBENV_VERSION. Fixes #121

This removes the workaround created for #15 and solves `ruby -S` support
by setting RUBYPATH. PATH is never changed.

To illustrate how RUBYPATH changes in various configurations:

    PATH=~/bin:~/.rbenv/shims:/usr/bin:/bin
    RBENV_VERSION=1.8 ruby -S rake
    #=> executes ~/.rbenv/versions/1.8/bin/rake
    #=> RUBYPATH=~/bin:~/.rbenv/versions/1.8/bin:/usr/bin:/bin

    RBENV_VERSION=2.0 ruby -S rake
    #=> executes ~/.rbenv/versions/2.0/bin/rake
    #=> RUBYPATH=~/bin:~/.rbenv/versions/2.0/bin:/usr/bin:/bin

    RBENV_VERSION=system ruby -S rake
    #=> executes /usr/bin/rake
    #=> RUBYPATH=~/bin:/rbenv_shims_were_here:/usr/bin:/bin

    RBENV_VERSION=1.8 ruby -S rake
    #=> executes ~/.rbenv/versions/1.8/bin/rake
    #=> RUBYPATH=~/bin:~/.rbenv/versions/1.8/bin:/usr/bin:/bin

this is because `hg` (which is a dependency of *-dev build) might be
installed in existing pyenv versions.

than compatibility plugin.

`--with-*-dir` is only effective for Ruby's mkmf.rb.

The `has_broken_mac_readline` checks the existence of
`readline/rhconf.h`. This is the same mechanism as setup.py of CPython.

https://bitbucket.org/pypa/setuptools/issue/14/installation-failed-on-jython

readlink comes from GNU coreutils.  On systems without it, rbenv used to
spin out of control when it didn't have readlink or greadlink available
because it would re-exec the frontend script over and over instead of the
worker script in libexec.

Fixes #389

Making empty commits in tests blocked by opening the default text
editor, waiting for a message which we specified to be empty.

http://thread.gmane.org/gmane.comp.version-control.git/225529

CPython 3.4.x requires "python3.4" to be executable during build
if it is in "$PATH".