Rename "overrides" to "custom."

This helps shift the emphasis from "replacing existing files" to "adding
missing files," without ruling either option out.

project/lib/custom/sdl/include/SDL_config.h

@@ -0,0 +1,55 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2020 Sam Lantinga <slouken@libsdl.org>
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+
+#ifndef SDL_config_h_
+#define SDL_config_h_
+
+#include "SDL_platform.h"
+
+/**
+ *  \file SDL_config.h
+ */
+
+/* Add any platform that doesn't build using the configure system. */
+#if defined(__WIN32__)
+#include "SDL_config_windows.h"
+#elif defined(__WINRT__)
+#include "SDL_config_winrt.h"
+#elif defined(__MACOSX__)
+#include "SDL_config_macosx.h"
+#elif defined(__IPHONEOS__)
+#include "SDL_config_iphoneos.h"
+#elif defined(__ANDROID__)
+#include "SDL_config_android.h"
+#elif defined(__PSP__)
+#include "SDL_config_psp.h"
+#elif defined(__OS2__)
+#include "SDL_config_os2.h"
+#elif defined(RASPBERRYPI)
+#include "SDL_config_rpi.h"
+#elif defined(HX_LINUX)
+#include "SDL_config_linux.h"
+#else
+/* This is a minimal configuration just to get SDL running on new platforms. */
+#include "SDL_config_minimal.h"
+#endif /* platform config */
+
+#endif /* SDL_config_h_ */