Make the intermediate release targets .PRECIOUS. (GoogleContainerTools#141)

Author: dlorenc

Makefile

@@ -53,6 +53,8 @@ $(BUILD_DIR)/$(PROJECT)-%-$(GOARCH): $(GO_FILES) $(BUILD_DIR)
 $(BUILD_DIR):
 mkdir -p $(BUILD_DIR)
 
+.PRECIOUS: $(foreach platform, $(SUPPORTED_PLATFORMS), $(BUILD_DIR)/$(PROJECT)-$(platform))
+
 .PHONY: cross
 cross: $(foreach platform, $(SUPPORTED_PLATFORMS), $(BUILD_DIR)/$(PROJECT)-$(platform).sha256)