10l: Correctly use preprocessor conditionals.

patch by Eddie Pang, eddpang gmail com

Originally committed as revision 12159 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/mem.h

@@ -29,10 +29,10 @@
 #ifdef __ICC
   #define DECLARE_ALIGNED(n,t,v)       t v __attribute__ ((aligned (n)))
   #define DECLARE_ASM_CONST(n,t,v)     const t __attribute__ ((aligned (n))) v
-#elif __GNUC__
+#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 _MSVC
+#elif defined(_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
 #else