pay attention to DESTDIR when building with NO_PERL_MAKEMAKER

pay attention to DESTDIR when building with NO_PERL_MAKEMAKER

Building with prefix=/some/where and temporarily installing it to
elsewhere for tar'ing up is done with:

    make prefix=/some/where
    make prefix=/some/where DESTDIR=/else/where install

Make handcrafted perl/perl.mak without NO_PERL_MAKEMAKER honour DESTDIR.

Ancient ExtUtils::MakeMaker (pre 6.11?) has the same issue, but recent
versions of Perl ships with at leat 6.17; this patch does not address that
issue.

Signed-off-by: Junio C Hamano <gitster@pobox.com>