Add CGO makefile flag

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

Author: crosbymichael

Makefile

@@ -23,6 +23,7 @@ DESTDIR ?= /usr/local
 VERSION=$(shell git describe --match 'v[0-9]*' --dirty='.m' --always)
 REVISION=$(shell git rev-parse HEAD)$(shell if ! git diff --no-ext-diff --quiet --exit-code; then echo .m; fi)
 PACKAGE=github.com/containerd/containerd
+SHIM_CGO_ENABLED ?= 0
 
 ifneq "$(strip $(shell command -v go 2>/dev/null))" ""
 	GOOS ?= $(shell go env GOOS)
@@ -184,15 +185,15 @@ bin/%: cmd/% FORCE
 
 bin/containerd-shim: cmd/containerd-shim FORCE # set !cgo and omit pie for a static shim build: https://github.com/golang/go/issues/17789#issuecomment-258542220
 	@echo "$(WHALE) bin/containerd-shim"
-	@CGO_ENABLED=0 go build ${GO_BUILD_FLAGS} -o bin/containerd-shim ${SHIM_GO_LDFLAGS} ${GO_TAGS} ./cmd/containerd-shim
+	@CGO_ENABLED=${SHIM_CGO_ENABLED} go build ${GO_BUILD_FLAGS} -o bin/containerd-shim ${SHIM_GO_LDFLAGS} ${GO_TAGS} ./cmd/containerd-shim
 
 bin/containerd-shim-runc-v1: cmd/containerd-shim-runc-v1 FORCE # set !cgo and omit pie for a static shim build: https://github.com/golang/go/issues/17789#issuecomment-258542220
 	@echo "$(WHALE) bin/containerd-shim-runc-v1"
-	@CGO_ENABLED=0 go build ${GO_BUILD_FLAGS} -o bin/containerd-shim-runc-v1 ${SHIM_GO_LDFLAGS} ${GO_TAGS} ./cmd/containerd-shim-runc-v1
+	@CGO_ENABLED=${SHIM_CGO_ENABLED} go build ${GO_BUILD_FLAGS} -o bin/containerd-shim-runc-v1 ${SHIM_GO_LDFLAGS} ${GO_TAGS} ./cmd/containerd-shim-runc-v1
 
 bin/containerd-shim-runc-v2: cmd/containerd-shim-runc-v2 FORCE # set !cgo and omit pie for a static shim build: https://github.com/golang/go/issues/17789#issuecomment-258542220
 	@echo "$(WHALE) bin/containerd-shim-runc-v2"
-	@CGO_ENABLED=0 go build ${GO_BUILD_FLAGS} -o bin/containerd-shim-runc-v2 ${SHIM_GO_LDFLAGS} ${GO_TAGS} ./cmd/containerd-shim-runc-v2
+	@CGO_ENABLED=${SHIM_CGO_ENABLED} go build ${GO_BUILD_FLAGS} -o bin/containerd-shim-runc-v2 ${SHIM_GO_LDFLAGS} ${GO_TAGS} ./cmd/containerd-shim-runc-v2
 
 binaries: $(BINARIES) ## build binaries
 	@echo "$(WHALE) $@"