release builds use less debug symbols and warnings

release build are triggered through either `make`,
or their specific target `make zstd-release` and `make lib-release`.
diff --git a/Makefile b/Makefile
index 14d1510..d86db7c 100644
--- a/Makefile
+++ b/Makefile
@@ -23,7 +23,7 @@
 endif
 
 .PHONY: default
-default: lib zstd
+default: lib zstd-release
 
 .PHONY: all
 all: allmost
@@ -50,6 +50,11 @@
 	@$(MAKE) -C $(PRGDIR) $@
 	cp $(PRGDIR)/zstd$(EXT) .
 
+.PHONY: zstd-release
+zstd-release:
+	@$(MAKE) -C $(PRGDIR)
+	cp $(PRGDIR)/zstd$(EXT) .
+
 .PHONY: zstdmt
 zstdmt:
 	@$(MAKE) -C $(PRGDIR) $@