Source: https://code.videolan.org/videolan/vlc/-/raw/3.0.18/contrib/src/upnp/libupnp-pthread-force.patch
--- upnp/configure.ac.pthread-w32      2018-05-17 11:51:47.115502500 +0200
+++ upnp/configure.ac  2018-05-17 12:49:30.371786100 +0200
@@ -667,6 +667,7 @@ ACX_PTHREAD(
 #
 # Update environment variables for pthreads
 #
+PTHREAD_LIBS="-lpthread -pthread"
 CC="$PTHREAD_CC"
 CFLAGS="$PTHREAD_CFLAGS $CFLAGS"
 LIBS="$PTHREAD_LIBS $LIBS"