Include protection for <unistd.h>, <sys/types.h> and <sys/stat.h>.

diff --git a/tools/makedep.c b/tools/makedep.c
index 2a2174a..bf51ba0 100644
--- a/tools/makedep.c
+++ b/tools/makedep.c
@@ -25,7 +25,9 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include <unistd.h>
+#ifdef HAVE_UNISTD_H
+# include <unistd.h>
+#endif
 
 /* Max first-level includes per file */
 #define MAX_INCLUDES 200