I'm actually working on an emacs library to work with JWT (JSON Web
Token) as described in RFC 7519 / 7797.  To achive that I must use an
variant of base 64 encoding named base64url as described in RFC4648.

Actually Emacs doesn't provide this variant.  I've attached an attempt
to implement it at C level.

I'm avare that my patch is far from perfect and I would welcome any
help or comments to improve it.

Thanks in advance.  Pierre.