simplify checks for enabling SSE2 code

also fixes build issues under vs11 which has a native arm compiler for
windows 8 targets

Change-Id: Id76c2deae9fc9de147d13ad0d34edffcb5a726c4

src/dsp/dec.c

@@ -715,7 +715,7 @@ void VP8DspInit(void) {
 
   // If defined, use CPUInfo() to overwrite some pointers with faster versions.
   if (VP8GetCPUInfo) {
-#if defined(__SSE2__) || defined(_MSC_VER)
+#if defined(WEBP_USE_SSE2)
     if (VP8GetCPUInfo(kSSE2)) {
       VP8DspInitSSE2();
     }