cmath: Use c99 math on a new enough msvcrt

MSVCRT 14+ supports the C99 math routines that we need.  Use them
accordingly.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@295509 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/cmath b/include/cmath
index 4efad61..e1b258b 100644
--- a/include/cmath
+++ b/include/cmath
@@ -440,7 +440,7 @@
 using ::nan;
 using ::nanf;
 
-#ifndef _LIBCPP_MSVCRT
+#if !(defined(_LIBCPP_MSVCRT) && ((_VC_CRT_NAJOR_VERSION-0) < 14))
 using ::nearbyint;
 using ::nearbyintf;
 using ::nextafter;
@@ -463,7 +463,7 @@
 using ::tgammaf;
 using ::trunc;
 using ::truncf;
-#endif // !_LIBCPP_MSVCRT
+#endif // !(defined(_LIBCPP_MSVCRT) && ((_VC_CRT_NAJOR_VERSION-0) < 14))
 
 using ::acosl;
 using ::asinl;
@@ -495,7 +495,7 @@
 
 using ::copysignl;
 
-#if !defined(_LIBCPP_MSVCRT) || (_VC_CRT_MAJOR_VERSION-0) >= 14
+#if !(defined(_LIBCPP_MSVCRT) && ((_VC_CRT_NAJOR_VERSION-0) < 14))
 using ::erfl;
 using ::erfcl;
 using ::exp2l;
@@ -526,7 +526,7 @@
 using ::scalbnl;
 using ::tgammal;
 using ::truncl;
-#endif // !defined(_LIBCPP_MSVCRT) || (_VC_CRT_MAJOR_VERSION-0) >= 14
+#endif // !(defined(_LIBCPP_MSVCRT) && ((_VC_CRT_NAJOR_VERSION-0) < 14))
 
 #if _LIBCPP_STD_VER > 14
 inline _LIBCPP_INLINE_VISIBILITY float       hypot(       float x,       float y,       float z ) { return sqrt(x*x + y*y + z*z); }