add Folly portability macros for C++17 inline variables

Summary: Add a portability macro for testing whether the compiler supports C++17's inline variables.

Reviewed By: yfeldblum

Differential Revision: D17957184

fbshipit-source-id: bff7d2bc15867e32f787860b4f8e5a0176ac1017

folly/Portability.h

@@ -507,6 +507,14 @@ constexpr auto kCpplibVer = 0;
 #define FOLLY_HAVE_NOEXCEPT_FUNCTION_TYPE 1
 #endif
 
+#if __cpp_inline_variables >= 201606L
+#define FOLLY_HAS_INLINE_VARIABLES 1
+#define FOLLY_INLINE_VARIABLE inline
+#else
+#define FOLLY_HAS_INLINE_VARIABLES 0
+#define FOLLY_INLINE_VARIABLE
+#endif
+
 // feature test __cpp_lib_string_view is defined in <string>, which is
 // too heavy to include here.  MSVC __has_include support arrived later
 // than string_view, so we need an alternate case for it.

folly/lang/CustomizationPoint.h

@@ -16,6 +16,7 @@
 
 #pragma once
 
+#include <folly/Portability.h>
 #include <folly/lang/StaticConst.h>
 
 //  FOLLY_DEFINE_CPO
@@ -31,7 +32,7 @@
 //  ensure that there is only a single instance of the CPO created and then a
 //  named reference in an anonymous namespace is required to avoid duplicate
 //  symbol definitions.
-#if __cpp_inline_variables >= 201606L
+#if FOLLY_HAS_INLINE_VARIABLES
 #define FOLLY_DEFINE_CPO(Type, Name) \
   namespace folly_cpo__ {            \
   inline constexpr Type Name{};      \