Replace unused CMAKE_COMPILER_IS_MINGW by supported MINGW (#2961)

 https://gitlab.kitware.com/cmake/cmake/-/merge_requests/6538

Related: https://github.com/urho3d/Urho3D/issues/2959

Source/ThirdParty/Assimp/code/CMakeLists.txt

@@ -481,7 +481,7 @@ ADD_ASSIMP_IMPORTER( IFC
 if (ASSIMP_BUILD_IFC_IMPORTER)
   if (MSVC)
     set_source_files_properties(IFCReaderGen1.cpp IFCReaderGen2.cpp PROPERTIES COMPILE_FLAGS "/bigobj")
-  elseif(CMAKE_COMPILER_IS_MINGW)
+  elseif(MINGW)
     set_source_files_properties(IFCReaderGen1.cpp IFCReaderGen2.cpp PROPERTIES COMPILE_FLAGS "-O2")   # Urho3D: Remove -Wa,-mbig-obj flag as some older MinGW version may not support it yet
   endif()
 endif (ASSIMP_BUILD_IFC_IMPORTER)

Source/ThirdParty/SDL/CMakeLists.txt

@@ -1371,7 +1371,7 @@ elseif(WINDOWS)
     if(HAVE_DINPUT_H)
       set(SDL_JOYSTICK_DINPUT 1)
       list(APPEND EXTRA_LIBS dinput8)
-      if(CMAKE_COMPILER_IS_MINGW)
+      if(MINGW)
         list(APPEND EXTRA_LIBS dxerr8)
       elseif (NOT USE_WINSDK_DIRECTX)
         list(APPEND EXTRA_LIBS dxerr)