Make sure that the tarball has everything.

* Makefile.am (distcheck-hook): New target.

Makefile.am

@@ -368,6 +368,20 @@ gen-ChangeLog:
 	rm -f $(distdir)/ChangeLog
 	mv $(distdir)/ChangeLog-t $(distdir)/ChangeLog
 
+distcheck-hook:
+	set -e;                                 \
+	manifest=$(distdir)/.tarball-manifest;  \
+	test -f "$$manifest";                   \
+	for x in `cat "$$manifest"`;            \
+	do                                      \
+	    if ! test -f $(distdir)/"$$x"       \
+                && ! test "$$x" = .gitignore;   \
+	    then                                \
+	        echo "Missing: $$x";            \
+	        exit 1;                         \
+	    fi;                                 \
+	done
+
 # Cleaning
 ##########