enforce C++17 at minimum

Summary: New blanket requirement: C++17 at minimum, across all compilers. Future changes will assume C++17. Enforce the requirement explicitly in order to catch outstanding violations and potential regressions more easily.

Reviewed By: Mizuchi, Orvid

Differential Revision: D52628290

fbshipit-source-id: 9442ef894ee416cbb330d9b68b8a3667582b88d0

README.md

@@ -10,7 +10,7 @@ Folly: Facebook Open-source Library
 <img src="static/logo.svg" alt="Logo Folly" width="15%" align="right" />
 
 Folly (acronymed loosely after Facebook Open Source Library) is a
-library of C++14 components designed with practicality and efficiency
+library of C++17 components designed with practicality and efficiency
 in mind. **Folly contains a variety of core library components used extensively
 at Facebook**. In particular, it's often a dependency of Facebook's other
 open source C++ efforts and place where those projects can share code.

folly/Portability.h

@@ -32,7 +32,7 @@
 #include <version>
 #endif
 
-static_assert(FOLLY_CPLUSPLUS >= 201402L, "__cplusplus >= 201402L");
+static_assert(FOLLY_CPLUSPLUS >= 201703L, "__cplusplus >= 201703L");
 
 #if defined(__GNUC__) && !defined(__clang__)
 static_assert(__GNUC__ >= 7, "__GNUC__ >= 7");