Fix MSVC identification, patch by Eddie Pang.

Originally committed as revision 12218 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/mem.h

@@ -32,7 +32,7 @@
 #elif defined(__GNUC__)
     #define DECLARE_ALIGNED(n,t,v)      t v __attribute__ ((aligned (n)))
     #define DECLARE_ASM_CONST(n,t,v)    static const t v attribute_used __attribute__ ((aligned (n)))
-#elif defined(_MSVC)
+#elif defined(_MSC_VER)
     #define DECLARE_ALIGNED(n,t,v)      __declspec(align(n)) t v
     #define DECLARE_ASM_CONST(n,t,v)    __declspec(align(n)) static const t v
 #else