CMake fixes for MingW (thanks, Ozkan!).

- ignore DXSDK_DIR for mingw environment
- use dxerr8 instead of dxerr for mingw.

Partially fixes Bugzilla #3016.

CMakeLists.txt

@@ -900,10 +900,12 @@ elseif(WINDOWS)
 
   # Check for DirectX
   if(DIRECTX)
+    if(NOT CMAKE_COMPILER_IS_MINGW)
       if("$ENV{DXSDK_DIR}" STREQUAL "")
         message_error("DIRECTX requires the \$DXSDK_DIR environment variable to be set")
       endif()
       set(CMAKE_REQUIRED_FLAGS "/I\"$ENV{DXSDK_DIR}\\Include\"")
+    endif()
     check_include_file(d3d9.h HAVE_D3D_H)
     check_include_file(d3d11_1.h HAVE_D3D11_H)
     check_include_file(ddraw.h HAVE_DDRAW_H)
@@ -914,10 +916,12 @@ 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 OR HAVE_XAUDIO2_H)
       set(HAVE_DIRECTX TRUE)
+      if(NOT CMAKE_COMPILER_IS_MINGW)
       # TODO: change $ENV{DXSDL_DIR} to get the path from the include checks
         link_directories($ENV{DXSDK_DIR}\\lib\\${PROCESSOR_ARCH})
         include_directories($ENV{DXSDK_DIR}\\Include)
       endif()
+    endif()
     set(CMAKE_REQUIRED_FLAGS ${ORIG_CMAKE_REQUIRED_FLAGS})
   endif()
 
@@ -1020,7 +1024,12 @@ elseif(WINDOWS)
     set(SOURCE_FILES ${SOURCE_FILES} ${JOYSTICK_SOURCES})
     if(HAVE_DINPUT_H)
       set(SDL_JOYSTICK_DINPUT 1)
-      list(APPEND EXTRA_LIBS dinput8 dxguid dxerr)
+      list(APPEND EXTRA_LIBS dinput8 dxguid)
+      if(CMAKE_COMPILER_IS_MINGW)
+        list(APPEND EXTRA_LIBS dxerr8)
+      else()
+        list(APPEND EXTRA_LIBS dxerr)
+      endif()
     endif()
     if(HAVE_XINPUT_H)
       set(SDL_JOYSTICK_XINPUT 1)