Work around Rubinius build script choking on RUBYLIB

Rubinius' `./configure` script is has a guard that aborts the build if RUBYLIB environment variable is set. As an unfortunate consequence, anybody using "rbenv-gem-rehash" plugin won't be able to build Rubinius.

This works around the issue by not writing to RUBYLIB if the name of the script to be executed is `./configure`. Granted, this might affect more projects than just Rubinius, but this assumes that a script named `./configure` won't install any gems that need to be rehashed afterwards.

This is a terrible thing to do, but I don't see a cleaner workaround.

Fixes #8. Also see rubinius/rubinius#2659