libstdc++: Fix use of feature test macro in <source_location>

libstdc++-v3/ChangeLog:

	* include/std/source_location: Check __glibcxx_contracts instead
	of __cpp_lib_contracts.
	* testsuite/18_support/contracts/includes.cc: New test.

libstdc++-v3/include/std/source_location

@@ -38,12 +38,9 @@
 namespace std
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
-#ifdef __cpp_lib_contracts
-  namespace contracts
-  {
-    class contract_violation;
-  }
-#endif // __cpp_lib_contracts
+#ifdef __glibcxx_contracts
+  namespace contracts { class contract_violation; }
+#endif
 
   /// A class that describes a location in source code.
   struct source_location
@@ -95,10 +92,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     constexpr source_location (const void *__t)
       : _M_impl (static_cast <const __impl*>(__t)) {}
 
-#ifdef __cpp_lib_contracts
+#ifdef __glibcxx_contracts
     /* To enable use of the source __impl*.  */
     friend class std::contracts::contract_violation;
-#endif // __cpp_lib_contracts
+#endif
   };
 
 _GLIBCXX_END_NAMESPACE_VERSION

libstdc++-v3/testsuite/18_support/contracts/includes.cc

@@ -0,0 +1,6 @@
+// { dg-options "-fcontracts" }
+// { dg-do compile { target c++26 } }
+
+// We should not get errors from including this before <contracts>:
+#include <source_location>
+#include <contracts>