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>

perl/Makefile

diff --git a/perl/Makefile b/perl/Makefile
index e3dd1a5..4ab21d6 100644
--- a/perl/Makefile
+++ b/perl/Makefile
@@ -29,11 +29,11 @@
 	'$(PERL_PATH_SQ)' -MError -e 'exit($$Error::VERSION < 0.15009)' || \
 	echo '	cp private-Error.pm blib/lib/Error.pm' >> $@
 	echo install: >> $@
-	echo '	mkdir -p "$(instdir_SQ)"' >> $@
-	echo '	$(RM) "$(instdir_SQ)/Git.pm"; cp Git.pm "$(instdir_SQ)"' >> $@
-	echo '	$(RM) "$(instdir_SQ)/Error.pm"' >> $@
+	echo '	mkdir -p "$$(DESTDIR)$(instdir_SQ)"' >> $@
+	echo '	$(RM) "$$(DESTDIR)$(instdir_SQ)/Git.pm"; cp Git.pm "$$(DESTDIR)$(instdir_SQ)"' >> $@
+	echo '	$(RM) "$$(DESTDIR)$(instdir_SQ)/Error.pm"' >> $@
 	'$(PERL_PATH_SQ)' -MError -e 'exit($$Error::VERSION < 0.15009)' || \
-	echo '	cp private-Error.pm "$(instdir_SQ)/Error.pm"' >> $@
+	echo '	cp private-Error.pm "$$(DESTDIR)$(instdir_SQ)/Error.pm"' >> $@
 	echo instlibdir: >> $@
 	echo '	echo $(instdir_SQ)' >> $@
 else