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/features2d/src/agast.cpp

@@ -45,7 +45,7 @@ The references are:
 #include "precomp.hpp"
 #include "agast_score.hpp"
 
-#ifdef _WIN32
+#ifdef _MSC_VER
 #pragma warning( disable : 4127 )
 #endif
 

modules/features2d/src/agast_score.cpp

@@ -44,7 +44,7 @@ The references are:
 
 #include "agast_score.hpp"
 
-#ifdef _WIN32
+#ifdef _MSC_VER
 #pragma warning( disable : 4127 )
 #endif