Do not unconditionally include unistd.h

configure.ac

@@ -212,7 +212,9 @@ AC_CHECK_TYPES([struct timespec],
 	       [], [
 #include <sys/types.h>
 #include <sys/stat.h>
+#ifdef HAVE_UNISTD_H
 #include <unistd.h>
+#endif
 #include <time.h>
 ])