Allow CGO_ENABLED to be overridden for build

On Mac OS they have some custom dns c library that uses some
configuration files other than resolv.conf to configure dns lookups. The
standard go library does not handle these custom configuration files
which causes dns lookups to fail for some mac users.

This allows the downstream pacakgers to override CGO_ENABLED to build
binaries that use the custom dns library.

Signed-off-by: Joe Julian <me@joejulian.name>

Makefile

@@ -18,12 +18,13 @@ ACCEPTANCE_DIR:=../acceptance-testing
 ACCEPTANCE_RUN_TESTS=.
 
 # go option
-PKG        := ./...
+PKG         := ./...
-TAGS       :=
+TAGS        :=
-TESTS      := .
+TESTS       := .
-TESTFLAGS  :=
+TESTFLAGS   :=
-LDFLAGS    := -w -s
+LDFLAGS     := -w -s
-GOFLAGS    :=
+GOFLAGS     :=
+CGO_ENABLED ?= 0
 
 # Rebuild the binary if any of these files change
 SRC := $(shell find . -type f -name '*.go' -print) go.mod go.sum
@@ -77,7 +78,7 @@ all: build
 build: $(BINDIR)/$(BINNAME)
 
 $(BINDIR)/$(BINNAME): $(SRC)
-	GO111MODULE=on CGO_ENABLED=0 go build $(GOFLAGS) -trimpath -tags '$(TAGS)' -ldflags '$(LDFLAGS)' -o '$(BINDIR)'/$(BINNAME) ./cmd/helm
+	GO111MODULE=on CGO_ENABLED=$(CGO_ENABLED) go build $(GOFLAGS) -trimpath -tags '$(TAGS)' -ldflags '$(LDFLAGS)' -o '$(BINDIR)'/$(BINNAME) ./cmd/helm
 
 # ------------------------------------------------------------------------------
 #  install