Define feature-test macro for std::enable_shared_from_this

	* include/bits/shared_ptr_base.h (__cpp_lib_enable_shared_from_this):
	Define feature-test macro.
	* testsuite/20_util/enable_shared_from_this/members/reinit.cc: Test
	for the macro.

From-SVN: r239094

libstdc++-v3/ChangeLog

 2016-08-03  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/bits/shared_ptr_base.h (__cpp_lib_enable_shared_from_this):
+	Define feature-test macro.
+	* testsuite/20_util/enable_shared_from_this/members/reinit.cc: Test
+	for the macro.
+
 	* include/bits/shared_ptr.h (shared_ptr::weak_type): Define.
 	* include/bits/shared_ptr_base.h (__shared_ptr::weak_type): Define.
 	* testsuite/20_util/shared_ptr/cons/43820_neg.cc: Adjust dg-error.

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

@@ -1472,6 +1472,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       void
       _M_assign(_Tp* __ptr, const __shared_count<_Lp>& __refcount) noexcept
       {
+#define __cpp_lib_enable_shared_from_this 201603
 	if (use_count() == 0)
 	  {
 	    _M_ptr = __ptr;

libstdc++-v3/testsuite/20_util/enable_shared_from_this/members/reinit.cc

@@ -20,6 +20,10 @@
 #include <memory>
 #include <testsuite_hooks.h>
 
+#if __cpp_lib_enable_shared_from_this < 201603
+# error "__cpp_lib_enable_shared_from_this < 201603"
+#endif
+
 struct X : public std::enable_shared_from_this<X> { };
 
 bool