Allow providing (additional) custom GO_LDFLAGS

Makefile: Allow providing `GO_LDFLAGS` externally (e.g. via export
of the variable in the build environment), which is what downstream
distributions use to build with their linker flags (e.g. for external
link mode and/or debug packages).
This prepends externally provided `GO_LDFLAGS` to the version
declaration.

Author: dvzrv

Diff for: Makefile

@@ -34,7 +34,7 @@ PROJECT := github.com/kubernetes-sigs/cri-tools
 BINDIR ?= /usr/local/bin
 
 VERSION ?= $(shell git describe --tags --dirty --always | sed 's/^v//')
-GO_LDFLAGS := -X $(PROJECT)/pkg/version.Version=$(VERSION)
+GO_LDFLAGS := $(GO_LDFLAGS) -X $(PROJECT)/pkg/version.Version=$(VERSION)
 
 BUILD_PATH := $(shell pwd)/build
 BUILD_BIN_PATH := $(BUILD_PATH)/bin/$(GOOS)/$(GOARCH)