bug#27049: [PATCH 2/2] gnu: Add js-mathjax.

[Arun Isaac]

These are WIP patches. Please provide feedback.

> +         (setenv "PATH" (string-join (map (match-lambda
> +                                            ((_ . input-path)
> +                                             (string-append input-path 
> "/bin")))
> +                                          %build-inputs)
> +                                     ":"))

It would be nice if this was handled by the trivial-build-system
itself. Almost all trivial-build-system packages I can think of need
some variant of this.

> +                (cond
> +                 ((string-match "\\.js$" file)
> +                  (mkdir-p (dirname install-path))
> +                  (system (format #f "uglify-js ~a > ~a" file install-path)))

I have to use `system' instead of `system*' here, because I need to make
use of ">" to redirect output to a file.

While minifying some files, an ascii decoding error is reported. I'm yet
to sort that out.