[wwwdocs] Add "-fexternal-blas" to gcc-4.3/changes.html

[FX Coudert]

> +    <a
> href="http://gcc.gnu.org/onlinedocs/gfortran/Code-Gen- 
> Options.html#Code-Gen-Options">
> +    -fexternal-blas</a></code> option has been added, which
> +    adds calls to BLAS for matrix operations such as
> +    <code>matmul</code> rather than using the build-in  
> algorithms.</li>
> +  </ul>

"build-in" should be "built-in", and I'd prefer "which generates  
calls to BLAS function for...", but I'm not a native english speaker.  
Brooks usually has wise things to say about wording...

FX