diff --git a/third-party/miniupnp/Makefile.am b/third-party/miniupnp/Makefile.am
index 93f337f17..809466466 100644
--- a/third-party/miniupnp/Makefile.am
+++ b/third-party/miniupnp/Makefile.am
@@ -1,6 +1,6 @@
 noinst_LIBRARIES = libminiupnp.a
 
-AM_CFLAGS = @PTHREAD_CFLAGS@ -DNDEBUG
+AM_CFLAGS = @PTHREAD_CFLAGS@ -DNDEBUG -D_GNU_SOURCE
 
 libminiupnp_a_SOURCES = \
     connecthostport.c \