Fix embarassing __GNU__ vs. __GNUC__ typo in preprocessor condition.

patch by Rafaël Carré, funman videolan org

Originally committed as revision 13652 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/mem.h

@@ -42,13 +42,13 @@
     #define DECLARE_ASM_CONST(n,t,v)    static const t v
 #endif
 
-#if defined(__GNUC__) && (__GNU__ > 3 || __GNU__ == 3 && __GNU_MINOR__ > 0)
+#if defined(__GNUC__) && (__GNUC__ > 3 || __GNUC__ == 3 && __GNUC_MINOR__ > 0)
     #define av_malloc_attrib __attribute__((__malloc__))
 #else
     #define av_malloc_attrib
 #endif
 
-#if defined(__GNUC__) && (__GNU__ > 4 || __GNU__ == 4 && __GNU_MINOR__ > 1)
+#if defined(__GNUC__) && (__GNUC__ > 4 || __GNUC__ == 4 && __GNUC_MINOR__ > 1)
     #define av_alloc_size(n) __attribute__((alloc_size(n)))
 #else
     #define av_alloc_size(n)