dsp.h: allow config.h to override MSVC SIMD autodetection

this fixes builds with cmake targeting visual studio that set
-DWEBP_ENABLE_SIMD=0

BUG=webp:478

Change-Id: I21b61b112c79ff9cbab9e4502a25d3f1fa096c8b

src/dsp/dsp.h

@@ -51,6 +51,7 @@ extern "C" {
 # define __has_builtin(x) 0
 #endif
 
+#if !defined(HAVE_CONFIG_H)
 #if defined(_MSC_VER) && _MSC_VER > 1310 && \
     (defined(_M_X64) || defined(_M_IX86))
 #define WEBP_MSC_SSE2  // Visual C++ SSE2 targets
@@ -60,6 +61,7 @@ extern "C" {
     (defined(_M_X64) || defined(_M_IX86))
 #define WEBP_MSC_SSE41  // Visual C++ SSE4.1 targets
 #endif
+#endif
 
 // WEBP_HAVE_* are used to indicate the presence of the instruction set in dsp
 // files without intrinsics, allowing the corresponding Init() to be called.
@@ -73,6 +75,9 @@ extern "C" {
 #define WEBP_USE_SSE41
 #endif
 
+#undef WEBP_MSC_SSE41
+#undef WEBP_MSC_SSE2
+
 // The intrinsics currently cause compiler errors with arm-nacl-gcc and the
 // inline assembly would need to be modified for use with Native Client.
 #if (defined(__ARM_NEON__) || \