Define __cpp_lib_byte feature-test macro

	* include/c_global/cstddef: Define __cpp_lib_byte feature-test macro.
	* testsuite/18_support/byte/requirements.cc: Check macro.

From-SVN: r253952

libstdc++-v3/ChangeLog

+2017-10-20  Jonathan Wakely  <jwakely@redhat.com>
+
+	* include/c_global/cstddef: Define __cpp_lib_byte feature-test macro.
+	* testsuite/18_support/byte/requirements.cc: Check macro.
+
 2017-10-19  Jonathan Wakely  <jwakely@redhat.com>
 
 	* testsuite/experimental/filesystem/iterators/

libstdc++-v3/include/c_global/cstddef

@@ -57,9 +57,11 @@ namespace std
 }
 #endif
 
-#if __cplusplus > 201402L
+#if __cplusplus >= 201703L
 namespace std
 {
+#define __cpp_lib_byte 201603
+
   /// std::byte
   enum class byte : unsigned char {};
 

libstdc++-v3/testsuite/18_support/byte/requirements.cc

@@ -20,6 +20,12 @@
 
 #include <cstddef>
 
+#ifndef __cpp_lib_byte
+# error "Feature-test macro for byte missing"
+#elif __cpp_lib_byte != 201603
+# error "Feature-test macro for byte has wrong value"
+#endif
+
 static_assert( sizeof(std::byte) == sizeof(unsigned char) );
 static_assert( alignof(std::byte) == alignof(unsigned char) );