Remove (untested) code for unsupported compilers.

src/base/win32-math.h

-// Copyright 2011 the V8 project authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-// Extra POSIX/ANSI routines for Win32 when using Visual Studio C++. Please
-// refer to The Open Group Base Specification for specification of the correct
-// semantics for these functions.
-// (http://www.opengroup.org/onlinepubs/000095399/)
-
-#ifndef V8_BASE_WIN32_MATH_H_
-#define V8_BASE_WIN32_MATH_H_
-
-#ifndef _MSC_VER
-#error Wrong environment, expected MSVC.
-#endif  // _MSC_VER
-
-// MSVC 2013+ provides implementations of all standard math functions.
-#if (_MSC_VER < 1800)
-enum {
-  FP_NAN,
-  FP_INFINITE,
-  FP_ZERO,
-  FP_SUBNORMAL,
-  FP_NORMAL
-};
-
-
-namespace std {
-
-int isfinite(double x);
-int isinf(double x);
-int isnan(double x);
-int isless(double x, double y);
-int isgreater(double x, double y);
-int fpclassify(double x);
-int signbit(double x);
-
-}  // namespace std
-
-#endif  // _MSC_VER < 1800
-
-#endif  // V8_BASE_WIN32_MATH_H_