COMP: Special case for MinGW and visibility.

If using MinGW then visibility support should not be used if we also use
the conventional Windows declspec logic. Thanks to Sascha Zelzer for
pointing this out.

CMake/vtkCompilerExtras.cmake

@@ -38,7 +38,7 @@ if(CMAKE_COMPILER_IS_GNUCXX)
   mark_as_advanced(VTK_USE_GCC_VISIBILITY)
 
   if(${_gcc_version} VERSION_GREATER 4.2.0 AND HAVE_GCC_VISIBILITY
-    AND VTK_USE_GCC_VISIBILITY AND BUILD_SHARED_LIBS)
+    AND VTK_USE_GCC_VISIBILITY AND BUILD_SHARED_LIBS AND NOT MINGW)
     # Should only be set if GCC is newer than 4.2.0
     set(VTK_ABI_CXX_FLAGS "-fvisibility=hidden -fvisibility-inlines-hidden")
   else()