Add macro AV_JOIN() for joining two tokens into one

Originally committed as revision 21482 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/avutil.h

@@ -29,6 +29,10 @@
 
 #define AV_STRINGIFY(s)         AV_TOSTRING(s)
 #define AV_TOSTRING(s) #s
+
+#define AV_GLUE(a, b) a ## b
+#define AV_JOIN(a, b) AV_GLUE(a, b)
+
 #define AV_PRAGMA(s) _Pragma(#s)
 
 #define AV_VERSION_INT(a, b, c) (a<<16 | b<<8 | c)