unexpected Make output (e.g. from --debug) causes build failure

Without this, the extra output produced e.g., by "make --debug"
would go into $INSTLIBDIR and then cause the sed command to fail.
Signed-off-by: NJim Meyering <jim@meyering.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Makefile

@@ -783,7 +783,7 @@ perl/perl.mak: GIT-CFLAGS
 
 $(patsubst %.perl,%,$(SCRIPT_PERL)): % : %.perl
 	$(QUIET_GEN)$(RM) $@ $@+ && \
-	INSTLIBDIR=`$(MAKE) -C perl -s --no-print-directory instlibdir` && \
+	INSTLIBDIR=`MAKEFLAGS= $(MAKE) -C perl -s --no-print-directory instlibdir` && \
 	sed -e '1{' \
 	    -e '	s|#!.*perl|#!$(PERL_PATH_SQ)|' \
 	    -e '	h' \