use xmm intrinsics for lrintf with x86_64-w64-mingw32

c.f.: http://git.xiph.org/?p=opus.git;a=commit;h=cd159fd1ec8ae64e6cd1b69854034560b5f1c419

--- opus-1.0.3/celt/float_cast.h~
+++ opus-1.0.3/celt/float_cast.h
@@ -61,7 +61,14 @@
 **      the config.h file.
 */
 
-#if (HAVE_LRINTF)
+#if (defined(__GNUC__) && defined(_WIN64))
+        #include <xmmintrin.h>
+        static __inline long int float2int(float value)
+        {
+                return _mm_cvtss_si32(_mm_load_ss(&value));
+        }
+
+#elif (HAVE_LRINTF)
 
 /*      These defines enable functionality introduced with the 1999 ISO C
 **      standard. They must be defined before the inclusion of math.h to