updates for MSVC 2015's degenerate headers

Note that 'perror' moves from stdio.h to stdlib.h, and 'rename' moves
from stdio.h to io.h. Also, standard C includes move from the compiler
to the Windows SDK, which changes the base path for the include files.

libtls-standalone/include/string.h

@@ -7,7 +7,11 @@
 #define LIBCRYPTOCOMPAT_STRING_H
 
 #ifdef _MSC_VER
+#if _MSC_VER >= 1900
+#include <../ucrt/string.h>
+#else
 #include <../include/string.h>
+#endif
 #else
 #include_next <string.h>
 #endif