There is a great new Emacs plugin from Eric Merritt that like FlyMake builds your code and highlights within Emacs any errors or warnings, but unlike FlyMake builds across the whole project. You can clone the mode from here projmake-mode
After cloning the repo to your desired location add this bit to your dot emacs file, replacing <PATH> with the path to where you cloned the repo.
This Emacs code also uses add-hook to set projmake-mode to start for erlang-mode is loaded. Projmake by default knows how to handle rebar and Make based builds so there is no setup after this, assuming your project is built this way.
Here is my Makefile for building Erlang projects with rebar, replace PROJECT with the name of your project:
Now you can load Emacs and a file from your project and if it is an Erlang file due to the add-hook function in our dot emacs file it will automatically load projmake-mode. You can add hooks for other modes or simply run M-x projmake-mode.
For more documentation and how to extend to other types of projects check out the documentation.