udp: fix compilation when HAVE_PTHREAD_CANCEL isnt defined

This happens with for example mplayer.
Fixing it in ffmpeg allows new ffmpeg to be compiled with older mplayer
which would not be possible if the fix was just in mplayer.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/udp.c

@@ -41,6 +41,10 @@
 #include <pthread.h>
 #endif
 
+#ifndef HAVE_PTHREAD_CANCEL
+#define HAVE_PTHREAD_CANCEL 0
+#endif
+
 #ifndef IPV6_ADD_MEMBERSHIP
 #define IPV6_ADD_MEMBERSHIP IPV6_JOIN_GROUP
 #define IPV6_DROP_MEMBERSHIP IPV6_LEAVE_GROUP