(Revised 11/5/2015 for MAME 0.167+ and the Apple II example now actually works).
make SUBTARGET=name SOURCES=src/mame/drivers/whatever.c REGENIE=1
name affects the output executable name (and can be omitted if the default 'mame' is OK), the SOURCES must be the partial path to at least one driver file.
For example, an Apple II-only binary can be made with "make SOURCES=src/mame/drivers/apple2.c,src/mame/drivers/apple2e.c,src/mame/video/apple2.c REGENIE=1". A Namco System 12-only binary can be made with "make SOURCES=src/mame/drivers/namcos12.c REGENIE=1".
This is much, much faster when working on a driver than making and linking all of MAME, especially post-merger. (Don't forget your usual -j flags to take advantage of multiple cores).
Edited by R. Belmont (11/05/15 12:34 PM)