fix #1941 darknet2ncnn maxpool out_w issue (#1972)

* fix #1941 darknet2ncnn maxpool out_w issue

* fix typo caused build fail

* fix opencv4 c++11 flags in example folder

examples/CMakeLists.txt

@@ -13,6 +13,9 @@ if(NOT OpenCV_FOUND)
 endif()
 
 if(OpenCV_FOUND)
+    if(${OpenCV_VERSION_MAJOR} GREATER_EQUAL 4)
+        set(CMAKE_CXX_STANDARD 11)
+    endif()
     include_directories(${CMAKE_CURRENT_SOURCE_DIR}/../src)
     include_directories(${CMAKE_CURRENT_BINARY_DIR}/../src)
 

tools/darknet/darknet2ncnn.cpp

@@ -472,7 +472,7 @@ void parse_cfg(std::deque<Section*>& dnet, int merge_output)
             s->w = p->out_w;
             s->c = p->out_c;
             s->out_h = (s->h + s->padding - s->size) / s->stride + 1;
-            s->out_w = s->out_h;
+            s->out_w = (s->w + s->padding - s->size) / s->stride + 1;
             s->out_c = s->c;
 
 #if OUTPUT_LAYER_MAP