quieten upstream's tar check

Index: Makefile
--- Makefile.orig
+++ Makefile
@@ -162,9 +162,9 @@ ZIP_V_1 =
 ZIP_V_2 =
 ZIP_V = $(ZIP_V_$(V))
 
-ifeq ($(shell tar --version | grep -c "GNU tar"),0)
+ifeq ($(shell tar --version 2>/dev/null | grep -c "GNU tar"),0)
 # Skip all flags if this is Darwin (a.k.a. macOS, a.k.a. OS X)
-ifeq ($(shell uname | grep -c "Darwin"),0)
+ifeq ($(shell uname | grep -c -e Darwin -e OpenBSD),0)
 TAR_FLAGS_FOR_REPRODUCIBLE_BUILDS = --uid 0 \
 				    --gid 0 \
 				    --numeric-owner \
