diff --git a/cpprules.in b/cpprules.in
index 7219e361f..2e0856bee 100644
--- a/cpprules.in
+++ b/cpprules.in
@@ -36,6 +36,7 @@ MANDEFS = \
 	-D__adminmansuffix__=$(ADMIN_MAN_SUFFIX) \
 	-D__mandir__=$(mandir) \
 	-D__projectroot__=$(prefix) \
+	-D__datadir__=$(datadir) \
 	-D__xconfigfile__=$(__XCONFIGFILE__) \
 	-D__xconfigdir__=$(__XCONFIGDIR__) \
 	-D__xkbdir__=$(XKB_BASE_DIRECTORY) \