Check that configuration variables are defined

book/support/makefiles/help.mk

@@ -16,3 +16,18 @@ help: ## Describe the main targets (this list)
 	@echo "Combined format+volume targets are also defined: pdfbook, htmlchapters…"
 	@echo "To make a single specific file/format, ask for it explicitly:"
 	@echo "  make $(OUTPUTDIRECTORY)/$(firstword $(CHAPTERS)).pdf"
+
+# Check that given variables are set and all have non-empty values,
+# die with an error otherwise.
+#
+# Params:
+#   1. Variable name(s) to test.
+#   2. (optional) Error message to print.
+#
+# See http://stackoverflow.com/questions/10858261/abort-makefile-if-variable-not-set
+check_defined = \
+		$(strip $(foreach 1,$1, \
+				$(call __check_defined,$1,$(strip $(value 2)))))
+__check_defined = \
+		$(if $(value $1),, \
+			$(error Undefined setting $1$(if $2, ($2))))

book/support/makefiles/prepare.mk

@@ -1,3 +1,7 @@
+$(call check_defined, OUTPUTDIRECTORY, Directory for build products)
+$(call check_defined, MAIN, Base name of the main document)
+$(call check_defined, CHAPTERS, Base names of the chapters)
+
 .phony: prepare prepare-build prepare-clean download submodules
 
 FIGURES := $(shell find . \