[patch, wwwdocs] Mention -fexternal-blas64 option

[Steve Kargl]

On Sun, Sep 21, 2025 at 06:23:13PM +0200, Thomas Koenig wrote:
> 
> the attached patch mentions -fexternal-blas64 in the changes.html file
> for gcc16. Checked with "tidy -e" and by visual inspection with Firefox.
> 
> Any comments or suggestions for better wording?  OK to commit?
> 

Wording looks fine to me.  Sorry about missing the -m32 issue.

-- 
Steve