[HACKERS] OpenSSL support in our back branches

I see that buildfarm member anchovy has been failing in pre-9.5
branches for the last several weeks, with symptoms suggesting that
it's been updated to openssl 1.1.0 or later.  The failures are
unsurprising given that commit 593d4e47d wasn't back-patched further
than 9.5, but I wonder whether it isn't time to do that now.

Alternatively, we could ask anchovy's owner to remove --with-openssl
from its configuration for pre-9.5 branches, but there are going to
be more and more buildfarm critters with this issue.
        regards, tom lane