syntax: Support &: literal (for make `grouped' multiple targets)

syntax: Support &: literal (for make `grouped' multiple targets)

Very recent versions of GNU make support this syntax:

  foo bar biz &: baz boz
        echo $^ > foo
        echo $^ > bar
        echo $^ > biz

(See (make)Multiple Targets, subheading `Rules with Grouped Targets')

This use of & is nonoverlapping with everything we care about, so we
can just pass it straight through.  Passing through &: means we pass
through &:: too.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>