check that the go stdlib was built with the required options

AFAICT, go build tags are a compile-time only construct, so there is
no way to query whether a certain package was built with a specific
tag. However, the 'netgo' tag triggers conditional compilation of the
cgo_stubs.go file, and that file name ends up in the executable. (we
could have looked for specific symbols in the 'net.a' library instead,
but that requires locating it first, which is more hassle than it is
worth)

issue #93

weaver/Makefile

@@ -8,6 +8,13 @@ all: docker-image
 weaver: ../*.go main.go
 	go get -tags netgo
 	go build -ldflags '-extldflags "-static"' -tags netgo
+	@strings $@ | grep cgo_stub\\\.go >/dev/null || { \
+		rm $@; \
+		echo "\nYour go standard library was built without the 'netgo' build tag."; \
+		echo "To fix that, run"; \
+		echo "    sudo go install -a -tags netgo std"; \
+		false; \
+	}
 
 .PHONY: docker-image
 docker-image: /var/tmp/weave.tar