don't use CMAKE_COMPILER_IS_MINGW

Fixes https://github.com/libsdl-org/SDL/issues/5002

CMakeLists.txt

@@ -1519,7 +1519,7 @@ elseif(WINDOWS)
     if(DEFINED MSVC_VERSION AND NOT ${MSVC_VERSION} LESS 1700)
         set(USE_WINSDK_DIRECTX TRUE)
     endif()
-    if(NOT CMAKE_COMPILER_IS_MINGW AND NOT USE_WINSDK_DIRECTX)
+    if(NOT MINGW AND NOT USE_WINSDK_DIRECTX)
       if("$ENV{DXSDK_DIR}" STREQUAL "")
         message_error("DIRECTX requires the \$DXSDK_DIR environment variable to be set")
       endif()
@@ -1537,7 +1537,7 @@ elseif(WINDOWS)
     check_include_file(dxgi.h HAVE_DXGI_H)
     if(HAVE_D3D_H OR HAVE_D3D11_H OR HAVE_DDRAW_H OR HAVE_DSOUND_H OR HAVE_DINPUT_H)
       set(HAVE_DIRECTX TRUE)
-      if(NOT CMAKE_COMPILER_IS_MINGW AND NOT USE_WINSDK_DIRECTX)
+      if(NOT MINGW AND NOT USE_WINSDK_DIRECTX)
       # TODO: change $ENV{DXSDL_DIR} to get the path from the include checks
         target_link_directories(sdl-build-options INTERFACE "$$ENV{DXSDK_DIR}\\lib\\${PROCESSOR_ARCH}")
         target_include_directories(sdl-build-options INTERFACE "$ENV{DXSDK_DIR}\\Include")