[RFC] PLIST feature to simplify conditionals

["Johnny C. Lam" <jlam%pkgsrc.org@localhost>]

I'd like to add a feature to the mk/plist module to make it simpler to 
do something that's quite common in pkgsrc.  Example usage:

        PLIST_VARS+=    nss pam

        .if !empty(PKG_OPTIONS:Mpam)
        PLIST.pam=      defined
        .endif

        .if !empty(PKG_OPTIONS:Mnss)
        PLIST.nss=      defined
        .endif

In the PLIST, you can then have:

${PLIST.pam}lib/security/pam_winbind.so
${PLIST.nss)lib/nss_winbind.so

Those PLIST.* variables automatically expand into "@comment " or an 
empty value in the PLIST processing depending on whether or not it's 
defined in the package Makefile.

Any comments on the variable names chosen, or the "API" presented?

        Thanks,

        -- Johnny C. Lam