Log Message:
net/nagios-plugin-spamd: Fix spamc embedded path

Upstream has a hard-coded path to spamc which is not correct in the
pkgsrc context.  Previously there was a SUBST block to change this,
but it was changed to an odd directory with a missing /, and it seems
this could never have worked.  Change the SUBST block to result in
looking for spamc in ${PREFIX}/bin/spamc, which is where the pkgsrc
build of spamassassin puts it.

Add comments explaining why there is both this SUBST block (embedded
spamc path) and REPLACE_PERL (interpreter path).

From Mike Pumford on pkgsrc-users.