math: actually pull the declarations/overloads into std

The previous changes missed the change to include/cmath.  These changes
allow some of the rand.distribution tests to pass on Windows.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@294957 91177308-0d34-0410-b5e6-96231b3b80d8

include/cmath

@@ -398,7 +398,7 @@ using ::cbrtf;
 using ::copysign;
 using ::copysignf;
 
-#ifndef _LIBCPP_MSVCRT
+#if !defined(_LIBCPP_MSVCRT) || ((_VC_CRT_NAJOR_VERSION-0) >= 14)
 using ::erf;
 using ::erff;
 using ::erfc;

include/math.h

@@ -293,6 +293,9 @@ long double    truncl(long double x);
 */
 
 #include <__config>
+#if defined(_LIBCPP_MSVCRT)
+#include <crtversion.h>
+#endif
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header