Open Builder, “Select a Folder” (middle button, bottom of greeter window).
Change the Filter (bottom right drop down of file chooser) to “Makefile”.
Browse to the directory containing your root Makefile and select it
Click “Open”
Note that there is no standard way to extract build flags from a Makefile so you’re unlikely to get many important features like clang integration for diagnostics/code-completion when using plain Makefiles.
There are not tools yet to modify build targets. It is something that is desired but a huge amount of work to abstract given the number of build systems we support. Contributions certainly desired on this front.