Make.rules: Define datarootdir to avoid a configure warning.
diff --git a/Make.rules.in b/Make.rules.in
index 9deee72..9af2d3e 100644
--- a/Make.rules.in
+++ b/Make.rules.in
@@ -98,6 +98,7 @@
 exec_prefix     = @exec_prefix@
 bindir          = @bindir@
 libdir          = @libdir@
+datarootdir     = @datarootdir@
 datadir         = @datadir@
 infodir         = @infodir@
 mandir          = @mandir@