Digging up my old "make standalone" stuff from a year ago:
http://busybox.net/lists/busybox/2005-September/015766.html
I renamed it "individual" to not confuse it with the standalone shell. (Which
it isn't compatible with for obvious reasons.) Configure busybox (I did
make defconfig), then run scripts/individual and it'll build an individual
version of each applet in the "build" subdirectory.
Currently it builds 146 and fails to build 104 applets out of "make defconfig".
I haven't taught it about multi-file applets yet (like tar), or the ones where
two applets get built from the same source (for example, zcat is a trivial
variant of gunzip so there is no zcat.c). But here's a start.