libstdc++: Fix preprocessor condition for inline variables

Although variable templates are valid in C++14, inline ones aren't.
These are only used in C++17 (or later) code, so they don't need to be
defined for C++14.

libstdc++-v3/ChangeLog:

	* include/bits/chrono.h (__is_duration_v, __is_time_point_v):
	Only define for C++17 and later.

diff --git a/libstdc++-v3/include/bits/chrono.h b/libstdc++-v3/include/bits/chrono.h
index b2e4f4c..fb99fe5 100644 (file)
--- a/libstdc++-v3/include/bits/chrono.h
+++ b/libstdc++-v3/include/bits/chrono.h
@@ -244,7 +244,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       using __disable_if_is_duration
        = typename enable_if<!__is_duration<_Tp>::value, _Tp>::type;
 
-#if __cpp_variable_templates
+#if __cplusplus >= 201703L
     template<typename _Tp>
       inline constexpr bool __is_duration_v = false;
     template<typename _Rep, typename _Period>