If anything in the.app/* changes, then rebuild the app:

APPS=$(wildcard *.app)
APPS_DEPS=$(patsubst %,%.dep,$(APPS))

%.dep: %/
        echo "vpath $*.scm $*" > $@
        find $* -type f -print0 | sed -e 's/ /\\ /g' | xargs -0 echo "$*.import.so: " >> $@
        find $* -type f -print0 | sed -e 's/ /\\ /g' | xargs -0 echo "$*.so: " >> $@