diff --git a/calendar/libical/src/libical/moz.build b/calendar/libical/src/libical/moz.build
index 87ba0e03f9..5e1d685465 100644
--- a/calendar/libical/src/libical/moz.build
+++ b/calendar/libical/src/libical/moz.build
@@ -36,3 +36,9 @@ FINAL_LIBRARY = 'xul'
 LOCAL_INCLUDES += [
     '../..'
 ]
+
+DEFINES['HAVE_CONFIG_H'] = True
+DEFINES['HAVE_SNPRINTF'] = True
+
+if CONFIG['OS_TARGET'] == 'Darwin':
+    DEFINES['__USE_BSD'] = '1'