In one case (buildroot), you are using a script that drives the whole build process (setting up the environment variables needed for the build process to use the cross-compiler/libraries instead of the build machine’s own build system) and moves everything around nicely. It is cool, but exactly tailored to do just that : fully building a plugin and prepare it for deployment.
You may just want to compile a different bit of code (for instance, @falkTX’s nooice module, that allows you to use a joystick or a guitar hero controller as an external midi controller), or to compile just one file or to do just a partial build, because you know your thing is not ready yet and that other parts of your plugin are going to cause the generation of zillions of messages that you don’t care about yet.
In that case, what you would like to have, is the ability to just use the traditional tools that you are used to on any other platform from the command line : gcc, make etc… instead of using the buildroot “black box” (well it is not really a black box, but its purpose is to be used like one).
This is what is called “Local”.
Put simply, you just need to source the right bash source file, which sets up all your environment nicely, and there you go. You are ready to do exactly what you expect : call make on your makefile that you want to test, compile just that one file for which you want to have all the warning messages, check which flags you need to use that particular lib, etc…
I hope this helps