[openssl-users] 'makedepend' in openssl builds: clarify need and correct usage

[PGNet Dev]

Actually, the actual admonition is more emphatic

> I'm prompted
>
>     Since you've disabled or enabled at least one algorithm, you need to do
>     the following before building:
>
>         make depend

"
Configured for linux-x86_64.

*** Because of configuration changes, you MUST do the following before
*** building:

         make depend
"