sendmail: fix build on hosts with Berkley DB installed

sendmail: fix build on hosts with Berkley DB installed

Buildbot caught an error:
  "Berkeley DB file locking needs flock() for version 5.x (and greater?)"

It is caused by leakage of host-installed Berkley DB into the build.
Since libmilter is not using the DB and because of convoluted build
process of sendmail, we do the workaround  - define a macro which
prevents the error without affecting libmilter binary.

Also change source URL from FTP to HTTPS.

Signed-off-by: Aleksey Vasilenko <redacted>