support building & cleaning extra dependencies
support building & cleaning extra dependencies

The app's Makefile can name targets in $(EXTRADEPS) and provide corresponding
rules to build them.

Also, hackish way to clean up in subdirectories, now named by $(SUBDIRS), is
recursive 'make clean'.

jck/accept,uwsampa/accept,uwsampa/accept,jck/accept,uwsampa/accept,jck/accept,jck/accept,uwsampa/accept