For the case where something isn't included automatically on a SOURCES= you just have to manually add that to the SOURCES. For instance, Apple II min-build looks like:

SOURCES=src/mame/drivers/apple2.cpp,src/mame/drivers/apple2e.cpp,src/mame/video/apple2.cpp

It doesn't (or didn't a few weeks ago at least) pick up the video/ file automatically.