* Maintain integrity of the substitute and successor mappings when

deleting a path in the store.
* Allow absolute paths in Nix expressions.
* Get nix-prefetch-url to work again.
* Various other fixes.

substitute.mk

@@ -1,9 +1,11 @@
 %: %.in Makefile
 	sed \
-	 -e s^@prefix\@^$(prefix)^g \
-	 -e s^@bindir\@^$(bindir)^g \
-	 -e s^@sysconfdir\@^$(sysconfdir)^g \
-	 -e s^@localstatedir\@^$(localstatedir)^g \
-	 -e s^@wget\@^$(wget)^g \
+	 -e "s^@prefix\@^$(prefix)^g" \
+	 -e "s^@bindir\@^$(bindir)^g" \
+	 -e "s^@sysconfdir\@^$(sysconfdir)^g" \
+	 -e "s^@localstatedir\@^$(localstatedir)^g" \
+	 -e "s^@datadir\@^$(datadir)^g" \
+	 -e "s^@host\@^$(host)^g" \
+	 -e "s^@wget\@^$(wget)^g" \
 	 < $< > $@ || rm $@
 	chmod +x $@