Keeping headers in the src directory makes sense for the interfaces we
don't want to expose outside, but the `include_lib` directive fails to
compile under Mix. Changing `include_lib("emqx_gateway/src/...")` to
`-include("src/...")` pleases both Mix and Rebar.
This commit enables a minimal working build of EMQX release using
Mix. However, to properly start the release, several configuration
steps are still missing. A `mix_release.sh` script does a few hacks
to get the release built with Mix to start properly, by first assuming
that `make emqx` has been run prior to the release, ran once to
generate the `app.*.config` files, and then it copies that and some
other files to the expected places.
Also, `emqx_telemetry` hangs the start procedure because it thinks
it's in an official release and tries to make a request. We disable
it temporarily via config just to get a working build for now.
JianBo He