Fix single-threaded build for targets without atomics

	* src/c++17/memory_resource.cc [!_GLIBCXX_HAS_GTHREADS]
	(atomic_mem_res): Add unsynchronized definition for single-threaded.

From-SVN: r263554

libstdc++-v3/ChangeLog

+2018-08-15  Jonathan Wakely  <jwakely@redhat.com>
+
+	* src/c++17/memory_resource.cc [!_GLIBCXX_HAS_GTHREADS]
+	(atomic_mem_res): Add unsynchronized definition for single-threaded.
+
 2018-08-14  Jonathan Wakely  <jwakely@redhat.com>
 
 	PR libstdc++/86954

libstdc++-v3/src/c++17/memory_resource.cc

@@ -88,7 +88,7 @@ namespace pmr
 #if ATOMIC_POINTER_LOCK_FREE == 2
     using atomic_mem_res = atomic<memory_resource*>;
 # define _GLIBCXX_ATOMIC_MEM_RES_CAN_BE_CONSTANT_INITIALIZED
-#else
+#elif defined(_GLIBCXX_HAS_GTHREADS)
     // Can't use pointer-width atomics, define a type using a mutex instead:
     struct atomic_mem_res
     {
@@ -114,6 +114,26 @@ namespace pmr
 	return std::exchange(val, r);
       }
     };
+#else
+# define _GLIBCXX_ATOMIC_MEM_RES_CAN_BE_CONSTANT_INITIALIZED
+    // Single-threaded, no need for synchronization
+    struct atomic_mem_res
+    {
+      constexpr
+      atomic_mem_res(memory_resource* r) : val(r) { }
+
+      memory_resource* val;
+
+      memory_resource* load() const
+      {
+	return val;
+      }
+
+      memory_resource* exchange(memory_resource* r)
+      {
+	return std::exchange(val, r);
+      }
+    };
 #endif // ATOMIC_POINTER_LOCK_FREE == 2
 
 #ifdef _GLIBCXX_ATOMIC_MEM_RES_CAN_BE_CONSTANT_INITIALIZED