diff --git a/configure b/configure
index cbb31bdf5a..0866a8bb75 100755
--- a/configure
+++ b/configure
@@ -974,7 +974,7 @@ enable stripping
 vhook="default"
 
 # build settings
-add_cflags '-D_ISOC9X_SOURCE'
+add_cflags '-D_ISOC99_SOURCE '
 SHFLAGS='-shared -Wl,-soname,$$(@F)'
 VHOOKSHFLAGS='$(SHFLAGS)'
 FFSERVERLDFLAGS=-Wl,-E