Clarify build test for presense of 'float copysign(float, float)'.

include/core/SkFloatingPoint.h

Index: include/core/SkFloatingPoint.h
diff --git a/include/core/SkFloatingPoint.h b/include/core/SkFloatingPoint.h
index 93d479cf8709e91138196854574d2f852ec02604..2df8f9bf65e258e6e5164f9d5182bfd378797d81 100644
--- a/include/core/SkFloatingPoint.h
+++ b/include/core/SkFloatingPoint.h
@@ -31,7 +31,9 @@ static inline float sk_float_pow(float base, float exp) {
 
 static inline float sk_float_copysign(float x, float y) {
 // c++11 contains a 'float copysign(float, float)' function in <cmath>.
-#if (!defined(_MSC_VER) && __cplusplus >= 201103L) || (defined(_MSC_VER) && _MSC_VER >= 1800)
+// clang-cl reports __cplusplus for clang, not the __cplusplus vc++ version _MSC_VER would report.
+#define SK_BUILD_WITH_CLANG_CL (defined(_MSC_VER) && defined(__clang__))
+#if (!SK_BUILD_WITH_CLANG_CL && __cplusplus >= 201103L) || (defined(_MSC_VER) && _MSC_VER >= 1800)
     return copysign(x, y);
 
 // Posix has demanded 'float copysignf(float, float)' (from C99) since Issue 6.