Update Makefile to return proper exit codes of sub-make commands.

Makefile

@@ -1,6 +1,11 @@
 # Convert relative Rack directory path to absolute
 RACK_DIR := $(shell realpath $(RACK_DIR))
 
+REPOS ?= $(shell ls repos)
 
-dist_all:
-	for f in repos/*; do $(MAKE) -C "$$f" dist; done
+dist_all: $(REPOS)
+
+$(REPOS):
+	$(MAKE) -C repos/$@ dist
+
+.PHONY: dist_all $(REPOS)