Utilize _BitScanReverse in Visual Studio

_BitScanReverse is documented here:
https://msdn.microsoft.com/en-us/library/fbxyd7zd.aspx
This improves speed of a VS2015 x64 build by around 8%.
It's slightly slower than __lzcnt, but compatible with old cpu.

src/zopfli/util.c

@@ -33,15 +33,22 @@ Author: [email protected] (Jyrki Alakuijala)
 /* __builtin_clz available beginning with GCC 3.4 */
 #elif __GNUC__ * 100 + __GNUC_MINOR__ >= 304
 # define HAS_BUILTIN_CLZ
+/* _BitScanReverse available beginning with Visual Studio 2005 */
+#elif _MSC_VER >= 1400
+# include <intrin.h> 
+# define HAS_BITSCANREVERSE
 #endif
 
 int ZopfliGetDistExtraBits(int dist) {
-#ifdef HAS_BUILTIN_CLZ
   if (dist < 5) return 0;
+#ifdef HAS_BUILTIN_CLZ
   return (31 ^ __builtin_clz(dist - 1)) - 1; /* log2(dist - 1) - 1 */
+#elif defined HAS_BITSCANREVERSE
+  unsigned long index;
+  _BitScanReverse(&index, dist - 1);
+  return index - 1;
 #else
-  if (dist < 5) return 0;
-  else if (dist < 9) return 1;
+  if (dist < 9) return 1;
   else if (dist < 17) return 2;
   else if (dist < 33) return 3;
   else if (dist < 65) return 4;