Allow gem flags to be set through environment variables

When building jruby, ruby-build automatically installs the jruby-launcher gem.

The problem is that I would like to specify that it should install it from a source I specify, rather than from rubygems.org. However, since I won't have access to the `gem` command until after jruby is installed, it is currently impossible for me to do that.

I have added two environment variables to allow users to inject command line parameters to `gem`, making this possible.

This may not be a need many people have but it is not an invasive change.

Thanks!
Mike