Enforce C++14 or later

Summary:
[Folly] Enforce C++14 or later.

Could alternatively do it via C++14-only syntax which would work for all target compilers, but this way the intention is clearest.

Reviewed By: mzlee

Differential Revision: D13649917

fbshipit-source-id: 0783dfd3e471d32626bbd44cc22584d83b5c21d4

folly/Portability.h

@@ -16,6 +16,12 @@
 
 #pragma once
 
+// MSCV 2017 __cplusplus definition by default does not track the C++ version.
+// https://devblogs.microsoft.com/cppblog/msvc-now-correctly-reports-__cplusplus/
+#if !defined(_MSC_VER) || _MSC_VER >= 2000
+static_assert(__cplusplus >= 201402L, "__cplusplus >= 201402L");
+#endif
+
 #include <cstddef>
 
 #include <folly/CPortability.h>