FTP,HTTP → HTTPS in Automake doc and code

[Paul Eggert]

I installed into Automake 'master' a patch to prefer https: URLs to ftp: and 
http: URLs. It's a long patch so I won't bore the mailing list with it. Details 
and justification are here:

https://git.savannah.gnu.org/cgit/automake.git/commit/?id=199e7a445040270fa5ef67623c56cde40d765199