[libc++] Limit gets to CRT versions below 14

[smeenai]

Summary

Microsoft removed gets from the CRT in Visual Studio 2015 onwards [1].
Attempting to reference it when targeting CRT versions 14 and above will
cause compile errors.

[1] https://msdn.microsoft.com/en-us/library/2029ea5f.aspx

[compnerd]

SVN r280417

include/cstdio

 int ferror(FILE* stream);
 void perror(const char* s);
 
 }  // std
 */
 
 #include <__config>
 #include <stdio.h>
+#if defined(_LIBCPP_MSVCRT)
+#include <crtversion.h>
+#endif
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 using ::FILE;

 using ::remove;
 using ::rename;
 using ::tmpfile;
 using ::tmpnam;
 #endif
 
 #ifndef _LIBCPP_HAS_NO_STDIN
 using ::getchar;
-#if _LIBCPP_STD_VER <= 11
+#if _LIBCPP_STD_VER <= 11 && \
+    (!defined(_VC_CRT_MAJOR_VERSION) || _VC_CRT_MAJOR_VERSION < 14)
 using ::gets;
 #endif
 using ::scanf;
 using ::vscanf;
 #endif
 
 #ifndef _LIBCPP_HAS_NO_STDOUT
 using ::printf;
 using ::putchar;
 using ::puts;
 using ::vprintf;
 #endif
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif  // _LIBCPP_CSTDIO