quieten upstream's tar check

Index: Makefile
--- Makefile.orig
+++ Makefile
@@ -314,9 +314,9 @@ $(SOURCE_DIST).manifest: $(SOURCE_DIST)
 	$(gen_verbose) cd $(dir $(SOURCE_DIST)) && \
 		find $(notdir $(SOURCE_DIST)) | LC_COLLATE=C sort > $@
 
-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 \