Fix handling of non ASCII values on ENV

If we set an environment variable with a non ASCII value and we set
Encoding.default_internal the program can crash.

Checking for ASCII values before we proceed to encode solves the
problem.

I don't like using a conditional in the spec, but if we are going to use
the env set by the user while running the spec suite we need to ignore
some of the values.
Set RBX_PREFIX_PATH to the build prefix when building extensions

When building rbx with the same path settings as an already-
installed rbx, it now uses the former and not the latter to build
the gems.

Important to have for packaging.
Merge pull request #3008 from heftig/master

Set RBX_PREFIX_PATH to the build prefix when building extensions
Creates --use-gemfile-git option to use git repos for runtime gems

This patch adds the --use-gemfile-git option which causes the
configure script to use the git repo directories in vendor/cache
directory when copying gems to the runtime/gems directory.

This can be used along with `bundle package --all` to use git
gems specified in the Gemfile instead of downloading the gems
from rubygems.org