"//go:build cgo".  Therefore, if cgo is disabled, files that import
"C" will not be built by the go tool. (For more about build constraints
see https://golang.org/pkg/go/build/#hdr-Build_Constraints).

When cross-compiling, you must specify a C cross-compiler for cgo to
use. You can do this by setting the generic CC_FOR_TARGET or the
more specific CC_FOR_${GOOS}_${GOARCH} (for example, CC_FOR_linux_arm)