Add $DH_GOLANG_GOPATH_ADD to pass a custom dir with build dependencies
During build, only libraries in /usr/share/gocode/src are symlinked in configure to the Go workspace to become available. $DH_GOLANG_GOPATH_ADD is a variable where one can pass any other directory to be symlinked into the build Go workspace.
This feature will be useful for people trying to build custom packages where not all dependencies are available in the default path for Golang packages in Debian.