fix linux compile

paddle/fluid/framework/ir/node.cc

@@ -17,8 +17,9 @@ limitations under the License. */
 namespace paddle {
 namespace framework {
 namespace ir {
+// msvc15 don't support constexpr in correct way.
 #if !defined(_WIN32)
-constexpr char Node::kControlDepVarName[] =  "__control_var";
+constexpr char Node::kControlDepVarName[];
 #else
 const char Node::kControlDepVarName[] = "__control_var";
 #endif