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/imgcodecs/src/grfmt_jpeg.cpp

@@ -213,7 +213,7 @@ ImageDecoder JpegDecoder::newDecoder() const
 
 bool  JpegDecoder::readHeader()
 {
-    bool result = false;
+    volatile bool result = false;
     close();
 
     JpegState* state = new JpegState;

modules/imgcodecs/src/grfmt_png.cpp

@@ -140,7 +140,7 @@ void  PngDecoder::readDataFromBuf( void* _png_ptr, uchar* dst, size_t size )
 
 bool  PngDecoder::readHeader()
 {
-    bool result = false;
+    volatile bool result = false;
     close();
 
     png_structp png_ptr = png_create_read_struct( PNG_LIBPNG_VER_STRING, 0, 0, 0 );