diff --git a/src/os_msdos.h b/src/os_msdos.h
index 49420a8..95578bc 100644
--- a/src/os_msdos.h
+++ b/src/os_msdos.h
@@ -35,6 +35,7 @@
 #define HAVE_MEMSET
 #define HAVE_QSORT
 #define HAVE_ST_MODE		/* have stat.st_mode */
+#define HAVE_MATH_H
 #if defined(__DATE__) && defined(__TIME__)
 # define HAVE_DATE_TIME
 #endif