Assume a GOBIN path

[chrissexton]

As a usual Go user, I do not have my GOBIN environment variable set and the makefile will bomb out when used vanilla and that's probably going to happen on usual configurations where the source is distributed.

Instead of bombing out with an error, it may be safe to assume GOBIN="$GOPATH/bin" when GOBIN is unset. This would make the Makefile act more in line with what the go tool does.