Fix SDL threading on macOS/Linux.

Author: bruvzg

drivers/sdl/SCsub

@@ -90,9 +90,6 @@ if env["builtin_sdl"]:
         "stdlib/SDL_string.c",
         "stdlib/SDL_strtokr.c",
         "thread/SDL_thread.c",
-        "thread/generic/SDL_syscond.c",
-        "thread/generic/SDL_sysrwlock.c",
-        "thread/generic/SDL_systhread.c",
         "timer/SDL_timer.c",
     ]
 
@@ -182,6 +179,8 @@ if env["builtin_sdl"]:
             "joystick/windows/SDL_windows_gaming_input.c",
             "joystick/windows/SDL_windowsjoystick.c",
             "joystick/windows/SDL_xinputjoystick.c",
+            "thread/generic/SDL_syscond.c",
+            "thread/generic/SDL_sysrwlock.c",
             "thread/windows/SDL_syscond_cv.c",
             "thread/windows/SDL_sysmutex.c",
             "thread/windows/SDL_sysrwlock_srw.c",