Input on "most correct" way to set IS_INTERACTIVE for Postfix ports

[Sahil Tandon]

mail/postfix and its derivatives are interactive when ALL of the
following conditions are true:

 - PACKAGE_BUILDING is undefined
 - /etc/mail/mailer.conf exists
 - /etc/mail/mailer.conf contains a line beginning with 'purgestat'

The way I plan to implement this in the port's Makefile is below; please
let me know if you notice errors or would suggest an another method.  I
appreciate that the use of '!=' is discouraged by portlint(1), and am
thusly open to cluebats and alternative suggestions.

 OLD_MAILER!=    ${GREP} -s '^purgestat' /etc/mail/mailer.conf || true

 .if !defined(PACKAGE_BUILDING) && !empty(OLD_MAILER)
 IS_INTERACTIVE= yes
 .endif

Thanks,
-- 
Sahil Tandon
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