diff --git a/configure.ac b/configure.ac
index d9be47dccc..75df6c75de 100644
--- a/configure.ac
+++ b/configure.ac
@@ -317,6 +317,7 @@ char * strerror(int n);
 #define BOOST_NO_WSTRING 1
 
 #ifdef __CYGWIN__
+#  define _DEFAULT_SOURCE
 #  define NOMINMAX
 #  define BOOST_POSIX 1
 #  define BOOST_POSIX_API 1