First attempt at replacement headers. Obviously a long way to go here…

…. :)

README.md

@@ -15,3 +15,22 @@ If you absolutely must have the real SDL 1.2 ("SDL 1.2 Classic"), please use
 the source tree at https://github.com/libsdl-org/SDL-1.2, which occasionally
 gets bug fixes but no formal release. But we strongly encourage you not to do
 that.
+
+# How to use:
+
+- Build the library. This will need access to SDL2's headers, CMake
+( https://cmake.org/ ) and the build tools of your choice. Once built, you
+will have a drop-in replacement that can be used with any existing binary
+that relies on SDL 1.2. You can copy this library over the existing 1.2 build,
+or force it to take priority over a system copy with LD_LIBRARY_PATH, etc.
+At runtime, sdl12-compat needs to be able to find a copy of SDL2, so plan to
+include it with the library if necessary.
+
+- If you want to build an SDL 1.2 program from source code, we have included
+compatibility headers, so that sdl12-compat can completely replace SDL 1.2
+at all points. These headers are just the bare minimum needed for source-level
+compatibility and don't have a lot of documentation or fanciness at all. The
+new headers are also under the zlib license. Note that sdl12-compat itself
+does not use these headers, so if you just want the library, you don't need
+them.
+

include/SDL/SDL.h

@@ -0,0 +1,32 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2019 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.
+*/
+
+#ifdef __BUILDING_SDL12_COMPAT__
+#error You should not use these headers to build sdl12-compat. Use the real SDL2 headers instead.
+#endif
+
+#ifndef _SDL_H
+#define _SDL_H
+
+#endif
+
+/* vi: set ts=4 sw=4 expandtab: */
+

src/SDL12_compat_objc.m

@@ -21,6 +21,7 @@
 
 /* This file contains some macOS-specific support code */
 
+#define __BUILDING_SDL12_COMPAT__ 1
 #include "SDL.h"
 
 #ifdef __MACOSX__

src/SDL20_include_wrapper.h

@@ -110,6 +110,7 @@
 #define INCL_DOSMODULEMGR /* for Dos_LoadModule() & co. */
 #endif
 
+#define __BUILDING_SDL12_COMPAT__ 1
 #include "SDL.h"
 #include "SDL_syswm.h"    /* includes windows.h for _WIN32, os2.h for __OS2__ */