Don't require the GIL for the empty constructor.

Cython/Compiler/ExprNodes.py

@@ -1692,7 +1692,8 @@ class NewExprNode(AtomicExprNode):
         self.cpp_check(env)
         constructor = type.scope.lookup(u'<init>')
         if constructor is None:
-            func_type = PyrexTypes.CFuncType(type, [], exception_check='+')
+            func_type = PyrexTypes.CFuncType(
+                type, [], exception_check='+', nogil=True)
             type.scope.declare_cfunction(u'<init>', func_type, self.pos)
             constructor = type.scope.lookup(u'<init>')
         self.class_type = type

tests/run/cpp_classes_def.pyx

@@ -143,3 +143,11 @@ def test_templates(long value):
     assert base.get_value() == base.value == 2 * value, base.value
 
     del base
+
+cdef cppclass Simple:
+  pass
+
+def test_default_init_no_gil():
+  with nogil:
+    s = new Simple()
+    del s