has not been supported by pkgsrc up to now.
This changes adds a PLIST macro @imake-man that expands to zero, one or
two lines, depending on the value of IMAKE_MANINSTALL. Packages must
explicitly have the macro in the PLIST files to use this feature. Since
currently no package has that, this change doesn't affect anything at
all by now.
TODO: The platforms' definitions for IMAKE_MANINSTALL and the other
definitions like IMAKE_MAN_SUFFIX need to be adjusted.
