Log Message:
Fix netbsd-5 build by defining shlib methods

Very surprisingly, postfix's build hard-codes shared library behavior
in a giant case statement not only per OS but per version, essentially
open-coding libtool while not being complete.  This commit copies the
netbsd-6 flags to netbsd-5, as a minimal change during the freeze to
let this build on netbsd-5 (where it then works fine).