mktables: Add code for easier handling of early Unicode versions

mktables: Add code for easier handling of early Unicode versions

This adds infrastructure to the constructor of the Input_file class to
allow an alternative to be specified when compiling a Unicode release
that is earlier than the file first became available.

This is only used when the property is used by core perl and has to work
in all releases.  For example the qr/\X/ construct should always work,
but relies on a property that isn't specified before Unicode 4.1.  This
allows for easier specification of how to handle this type of case.