Mark MSVC compiler macros as such.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@11918 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavutil/mem.h

@@ -32,7 +32,7 @@
 #elif __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)))
-#else
+#elif _MSVC
   #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
 #endif