diff --git a/configure b/configure
index a5d6e8d6..121cf6a 100755
--- a/configure
+++ b/configure
@@ -6694,6 +6694,8 @@
 	    SCO_SV*) OPT="$OPT -m486 -DSCO5"
 	    ;;
         esac
+
+        OPT="$OPT -std=c99"
 	;;
 
     *)
diff --git a/configure.ac b/configure.ac
index ec6d61b..3f72369 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1458,6 +1458,8 @@
 	    SCO_SV*) OPT="$OPT -m486 -DSCO5"
 	    ;;
         esac
+
+        OPT="$OPT -std=c99"
 	;;
 
     *)