Update check for VS2015
MODIFY
include/vcpkg/base/files.h
include/vcpkg/base/files.h
namespace fs
{
-#if defined(_WIN32) || defined(__cpp_lib_filesystem)
+// VS2015 (_MSC_VER 1900) uses std::experimental::filesystem
+#if (defined(_MSC_VER) && _MSC_VER > 1900) || defined(__cpp_lib_filesystem)
namespace stdfs = std::filesystem;
#else
namespace stdfs = std::experimental::filesystem;

Update check for VS2015