Pod-Perldoc: add Makefile.PL from distro

Since 3.21, the Makefile includes special code for copying perldoc.pod to
the right location, so use that rather than an auto-generated one. See

    [perl #120280] 5.19.5 intermittent failure t/search50.t

With this change, cpan/Pod-Perldoc/perldoc.pod is now copied to

    lib/perldoc.pod
rather than

    lib/Pod/perldoc.pod

(Its install location of lib/$version/pod/perldoc.pod is unaffected)