cmath: account for MSVCRT 12.0 changes

[abdulras]

Summary

MSVCRT 12.0 introduces better compatibility for C99. This includes a number of
math routines that were previously undefined. Use the crtversion.h header to
detect the version of MSVCRT being targeted and avoid re-declaring the
variables.

Since copysign has been introduced in MSVCRT, importing the definition via using
makes it difficult to provide overloads (due to minor differences between
throw () and noexcept. Avoid defining the overloads on newer MSVCRT
targets.

[mclow.lists]

This looks OK to me.

[abdulras]

SVN r230867.

include/cmath

 cbrt(_A1 __lcpp_x) _NOEXCEPT {return cbrt((double)__lcpp_x);}
 #endif
 
 // copysign
 
 using ::copysign;
 using ::copysignf;
 
-inline _LIBCPP_INLINE_VISIBILITY float       copysign(float __lcpp_x, float __lcpp_y) _NOEXCEPT             {return copysignf(__lcpp_x, __lcpp_y);}
-inline _LIBCPP_INLINE_VISIBILITY long double copysign(long double __lcpp_x, long double __lcpp_y) _NOEXCEPT {return copysignl(__lcpp_x, __lcpp_y);}
+#if !defined(_VC_CRT_MAJOR_VERSION) || (_VC_CRT_MAJOR_VERSION < 12)
+inline _LIBCPP_INLINE_VISIBILITY float copysign(float __lcpp_x,
+                                                float __lcpp_y) _NOEXCEPT {
+  return copysignf(__lcpp_x, __lcpp_y);
+}
+inline _LIBCPP_INLINE_VISIBILITY long double
+copysign(long double __lcpp_x, long double __lcpp_y) _NOEXCEPT {
+  return copysignl(__lcpp_x, __lcpp_y);
+}
+#endif
 
 template <class _A1, class _A2>
 inline _LIBCPP_INLINE_VISIBILITY
 typename __lazy_enable_if
 <
     is_arithmetic<_A1>::value &&
     is_arithmetic<_A2>::value,
     __promote<_A1, _A2>

include/support/win32/math_win32.h

 #define _LIBCPP_SUPPORT_WIN32_MATH_WIN32_H
 
 #if !defined(_LIBCPP_MSVCRT)
 #error "This header complements Microsoft's C Runtime library, and should not be included otherwise."
 #else
 
 #include <math.h>
 #include <float.h> // _FPCLASS_PN etc.
+#include <crtversion.h>
 
+#if ((_VC_CRT_MAJOR_VERSION-0) < 12)
 // Necessary?
 typedef float float_t;
 typedef double double_t;
 
 _LIBCPP_ALWAYS_INLINE bool isfinite( double num )
 {
     return _finite(num) != 0;
 }

 _LIBCPP_ALWAYS_INLINE double copysignl( long double x, long double y )
 {
     return ::_copysignl(x,y);
 }
 _LIBCPP_ALWAYS_INLINE int fpclassify( double num )
 {
     return _fpclass(num);
 }
-
+#endif
 #endif // _LIBCPP_MSVCRT
 
 #endif // _LIBCPP_SUPPORT_WIN32_MATH_WIN32_H