Guard use of concepts with feature test macro

This fixes a regression when using Clang.

	* include/bits/range_cmp.h: Check __cpp_lib_concepts before defining
	concepts. Fix comment.

From-SVN: r277449

libstdc++-v3/ChangeLog

+2019-10-25  Jonathan Wakely  <jwakely@redhat.com>
+
+	* include/bits/range_cmp.h: Check __cpp_lib_concepts before defining
+	concepts. Fix comment.
+
 2019-10-25  Gerald Pfeifer  <gerald@pfeifer.com>
 
 	* doc/xml/manual/policy_data_structures_biblio.xml: Switch

libstdc++-v3/include/bits/range_cmp.h

@@ -22,7 +22,7 @@
 // see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
 // <http://www.gnu.org/licenses/>.
 
-/** @file bits/ranges_function.h
+/** @file bits/range_cmp.h
  *  This is an internal header file, included by other library headers.
  *  Do not attempt to use it directly. @headername{functional}
  */
@@ -54,6 +54,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     using is_transparent = __is_transparent;
   };
 
+#ifdef __cpp_lib_concepts
 namespace ranges
 {
   namespace __detail
@@ -182,6 +183,7 @@ namespace ranges
   };
 
 } // namespace ranges
+#endif // library concepts
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace std
 #endif // C++20