Fixed few MinGW build problems

cmake/OpenCVDetectPython.cmake

-if(MSVC AND NOT PYTHON_EXECUTABLE)
+if(WIN32 AND NOT PYTHON_EXECUTABLE)
   # search for executable with the same bitness as resulting binaries
   # standard FindPythonInterp always prefers executable from system path
   # this is really important because we are using the interpreter for numpy search and for choosing the install location

cmake/OpenCVUtils.cmake

@@ -11,7 +11,7 @@ if(NOT COMMAND find_host_program)
   endmacro()
 endif()
 
-#added include directories in such way that directories from the OpenCV source tree go first
+# adds include directories in such way that directories from the OpenCV source tree go first
 function(ocv_include_directories)
   set(__add_before "")
   foreach(dir ${ARGN})
@@ -25,6 +25,12 @@ function(ocv_include_directories)
   include_directories(BEFORE ${__add_before})
 endfunction()
 
+# clears all passed variables
+macro(ocv_clear_vars)
+  foreach(_var ${ARGN})
+    unset(${_var} CACHE)
+  endforeach()
+endmacro()
 
 # Provides an option that the user can optionally select.
 # Can accept condition to control when option is available for user.
@@ -352,10 +358,11 @@ macro(ocv_parse_header2 LIBNAME HDR_PATH VARNAME SCOPE)
       set(${LIBNAME}_VERSION_STRING "${${LIBNAME}_VERSION_STRING}.${${LIBNAME}_VERSION_TWEAK}" ${SCOPE})
     endif()
   else()
-    unset(${LIBNAME}_VERSION_MAJOR CACHE)
-    unset(${LIBNAME}_VERSION_MINOR CACHE)
-    unset(${LIBNAME}_VERSION_PATCH CACHE)
-    unset(${LIBNAME}_VERSION_TWEAK CACHE)
-    unset(${LIBNAME}_VERSION_STRING CACHE)
+    ocv_clear_vars(${LIBNAME}_VERSION_MAJOR
+                   ${LIBNAME}_VERSION_MAJOR
+                   ${LIBNAME}_VERSION_MINOR
+                   ${LIBNAME}_VERSION_PATCH
+                   ${LIBNAME}_VERSION_TWEAK
+                   ${LIBNAME}_VERSION_STRING)
   endif()
 endmacro()

modules/highgui/src/window_w32.cpp

@@ -978,6 +978,11 @@ CV_IMPL int cvNamedWindow( const char* name, int flags )
     DWORD defStyle = WS_VISIBLE | WS_MINIMIZEBOX | WS_MAXIMIZEBOX | WS_SYSMENU;
     int len;
     CvRect rect;
+#ifdef HAVE_OPENGL
+    bool useGl;
+    HDC hGLDC;
+    HGLRC hGLRC;
+#endif
 
     cvInitSystem(0,0);
 
@@ -1013,9 +1018,9 @@ CV_IMPL int cvNamedWindow( const char* name, int flags )
     if (flags & CV_WINDOW_OPENGL)
         CV_ERROR( CV_OpenGlNotSupported, "Library was built without OpenGL support" );
 #else
-    bool useGl = false;
-    HDC hGLDC = 0;
-    HGLRC hGLRC = 0;
+    useGl = false;
+    hGLDC = 0;
+    hGLRC = 0;
 
     if (flags & CV_WINDOW_OPENGL)
         createGlContext(hWnd, hGLDC, hGLRC, useGl);