Make: Dynamic Makefile targets

A common pattern which we want to support when building our services is to build everything under rpc/{service}/{service}.proto as well as cmd/* and other locations which may be manually written or generated with various tools.

Building multiple targets with Go

Go does actually support building the whole cmd/ folder in one command:

go build -o ./build ./cmd/...

This functionality was added in Go just recently, in the 1.13.4 tag.

It will not create the target folder itself, but will build all...