set USE_DX9 in CMakeLists.txt instead of build.h

CMakeLists.txt

@@ -1171,6 +1171,7 @@ if(WIN32 AND NOT LIBRETRO AND NOT WINDOWS_STORE)
 	endif()
 
 	target_link_libraries(${PROJECT_NAME} PUBLIC d3d9 d3dx9)
+	target_compile_definitions(${PROJECT_NAME} PRIVATE USE_DX9)
 endif()
 
 if(WIN32)

core/build.h

@@ -222,9 +222,6 @@
 #if defined(WINAPI_FAMILY) && (WINAPI_FAMILY == WINAPI_FAMILY_APP)
 #define TARGET_UWP
 #endif
-#if !defined(LIBRETRO) && !defined(TARGET_UWP)
-#define USE_DX9
-#endif
 #endif