groups: added `make check'

This one has became direly needed while reworking
Centaurus' pkglists/groups into a tree.

pkg.in/groups/Makefile

@@ -2,8 +2,35 @@
 #         (and only those!) over to $(BUILDDIR)
 
 ifndef BUILDDIR
-$(error BUILDDIR not defined)
-endif
+
+check all: check-parents check-echo check-pkglists; @:
+
+check-parents:
+	@find -name "*.directory" \
+	| xargs -r -- sed -rn 's,^X-Alterator-Parent=(.*)$$,\1,p' \
+	| sort -u \
+	| while read group; do \
+		if ! [ -f "$$group.directory" ]; then \
+			echo "** Error: missing parent group referenced in:"; \
+			grep -rl "=$$group$$" $$(find -name "*.directory"); \
+		fi >&2; \
+	done
+
+check-echo:
+	@echo
+
+check-pkglists:
+	@find -name "*.directory" \
+	| xargs -r -- sed -rn 's,^X-Alterator-PackageList=(.*)$$,\1,p' \
+	| sort -u \
+	| while read list; do \
+		if ! [ -f "../lists/$$list" ]; then \
+			echo "** Error: missing pkglist referenced in:"; \
+			grep -rl "=$$list$$" $$(find -name "*.directory"); \
+		fi >&2; \
+	done
+
+else
 
 include $(BUILDDIR)/distcfg.mk
 SUFFIX := pkg/groups
@@ -20,3 +47,5 @@ all: $(GLOBAL_DEBUG)
 debug:
 	@echo "** THE_GROUPS: $(THE_GROUPS)"
 	@echo "** MAIN_GROUPS: $(MAIN_GROUPS)"
+
+endif