Some changes to support mingw-w64

- IPP is disabled by default when compiler is mingw (couldn't make it
work)
- fixed some warnings
- fixed some `__GNUC__` version checks (for correctness and convenience)
- removed UTF-8 BOM from hough.cpp (fixes #5253)

modules/highgui/src/window_w32.cpp

@@ -2036,8 +2036,8 @@ icvCreateTrackbar( const char* trackbar_name, const char* window_name,
     trackbar = icvFindTrackbarByName(window,trackbar_name);
     if( !trackbar )
     {
-        TBBUTTON tbs = {0};
-        TBBUTTONINFO tbis = {0};
+        TBBUTTON tbs = {};
+        TBBUTTONINFO tbis = {};
         RECT rect;
         int bcount;
         int len = (int)strlen( trackbar_name );