diff --git a/configure b/configure
index 6c5f949..0fba7f3 100755
--- a/configure
+++ b/configure
@@ -391,9 +391,9 @@ if [ "$debug" != "yes" -a "$gprof" != "yes" ]; then
 fi
 
 if [ "$debug" = "yes" ]; then
-    CFLAGS="-O1 -g $CFLAGS"
+    CFLAGS="-O1 -g -D_DEBUG $CFLAGS"
 else
-    CFLAGS="-O4 -ffast-math $CFLAGS"
+    CFLAGS="-O4 -ffast-math -DNDEBUG $CFLAGS"
 fi
 
 if cc_check "stdio.h" "" "fseeko(stdin,0,0);" ; then