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

include/core/SkFloatingPoint.h

 
 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #ifndef SkFloatingPoint_DEFINED
@@ skipping 13 matching lines @@
 
 // C++98 cmath std::pow seems to be the earliest portable way to get float pow.
 // However, on Linux including cmath undefines isfinite.
 // http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14608
 static inline float sk_float_pow(float base, float exp) {
     return powf(base, 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.
 #elif defined(_POSIX_VERSION) && _POSIX_VERSION >= 200112L
     return copysignf(x, y);
 
 // Visual studio prior to 13 only has 'double _copysign(double, double)'.
 #elif defined(_MSC_VER)
     return (float)_copysign(x, y);
 
@@ skipping 107 matching lines @@
     float estimate = *SkTCast<float*>(&i);
 
     // One step of Newton's method to refine.
     const float estimate_sq = estimate*estimate;
     estimate *= (1.5f-0.5f*x*estimate_sq);
     return estimate;
 #endif
 }
 
 #endif