Update value of __cpp_lib_chrono feature-test macro

	* include/std/chrono (__cpp_lib_chrono): Update macro value to
	indicate support for P0505R0.
	* testsuite/20_util/duration/arithmetic/constexpr_c++17.cc: Check
	for updated macro.

From-SVN: r253959

libstdc++-v3/ChangeLog

 2017-10-20  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/std/chrono (__cpp_lib_chrono): Update macro value to
+	indicate support for P0505R0.
+	* testsuite/20_util/duration/arithmetic/constexpr_c++17.cc: Check
+	for updated macro.
+
 	* include/c_global/cstddef: Define __cpp_lib_byte feature-test macro.
 	* testsuite/18_support/byte/requirements.cc: Check macro.
 

libstdc++-v3/include/std/chrono

@@ -214,8 +214,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
         treat_as_floating_point<_Rep>::value;
 #endif // C++17
 
-#if __cplusplus > 201402L
-# define __cpp_lib_chrono 201510
+#if __cplusplus >= 201703L
+# define __cpp_lib_chrono 201611
 
     template<typename _ToDur, typename _Rep, typename _Period>
       constexpr __enable_if_is_duration<_ToDur>

libstdc++-v3/testsuite/20_util/duration/arithmetic/constexpr_c++17.cc

@@ -20,6 +20,13 @@
 
 #include <chrono>
 #include <testsuite_common_types.h>
+
+#ifndef __cpp_lib_chrono
+# error "Feature-test macro for constexpr <chrono> missing"
+#elif __cpp_lib_chrono != 201611
+# error "Feature-test macro for constexpr <chrono> has wrong value"
+#endif
+
 constexpr auto test_operators()
 {
   std::chrono::nanoseconds d1 { 1 };