ensure MOREFLAGS are not lost in root->tests Makefile invocation

Author: Cyan4973

Makefile

@@ -77,7 +77,7 @@ check: shortest
 
 .PHONY: test shortest
 test shortest:
-	$(MAKE) -C $(PRGDIR) allVariants MOREFLAGS="-g -DZSTD_DEBUG=1"
+	$(MAKE) -C $(PRGDIR) allVariants MOREFLAGS+="-g -DZSTD_DEBUG=1"
 	$(MAKE) -C $(TESTDIR) $@
 
 .PHONY: examples

tests/Makefile

@@ -98,7 +98,7 @@ zstd-nolegacy:
 	$(MAKE) -C $(PRGDIR) $@ MOREFLAGS+="$(DEBUGFLAGS)"
 
 gzstd:
-	$(MAKE) -C $(PRGDIR) zstd HAVE_ZLIB=1 MOREFLAGS="$(DEBUGFLAGS)"
+	$(MAKE) -C $(PRGDIR) zstd HAVE_ZLIB=1 MOREFLAGS+="$(DEBUGFLAGS)"
 
 .PHONY:
 zstd-dll :