DESTDIR is supposed to be overridden on 'make install' after doing
'make'. Have the automatically generated perl/perl.mak not cache the
value of DESTDIR to support that for the perl/ subdirectory also.
Signed-off-by: Gerrit Pape <pape@smarden.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>