Test for presence of preflight and setup scripts

[kcranston]

@OrdiNeu make the following (good) suggestion: We could just test for the existence of the _preflight/_setup files during make compose-% before sourcing them, which would silence the file-not-found error message during compose like so:

	-if [ -f 'lib/$*/$*_preflight.sh' ]; then \
		source lib/$*/$*_preflight.sh 2>&1 | tee -a $(ERRORLOG); \
	fi