diff --git a/m4/mktime.m4 b/m4/mktime.m4
index c82183024..45d899af4 100644
--- a/m4/mktime.m4
+++ b/m4/mktime.m4
@@ -26,6 +26,10 @@ changequote(<<, >>)dnl
 # endif
 #endif
 
+#if HAVE_UNISTD_H
+# include <unistd.h>
+#endif
+
 #if !HAVE_ALARM
 # define alarm(X) /* empty */
 #endif