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/core/src/system.cpp

@@ -1191,6 +1191,9 @@ TLSData<CoreTLSData>& getCoreTlsData()
     #pragma warning(disable:4447) // Disable warning 'main' signature found without threading model
 #endif
 
+extern "C"
+BOOL WINAPI DllMain(HINSTANCE, DWORD fdwReason, LPVOID lpReserved);
+
 extern "C"
 BOOL WINAPI DllMain(HINSTANCE, DWORD fdwReason, LPVOID lpReserved)
 {