CMake SDL2 include paths missing on macOS

## Setup:

m1 MacBook Pro

macOS 13.0

SDL2.framework installed to /Library/Frameworks



## Repro:

build with the following commands run from the root of a clean copy of the repo

```

cmake -B ./build -DCMAKE_BUILD_TYPE=Debug

cmake --build ./build --config Debug

```



results in:

```

...

[ 60%] Building CXX object RecastDemo/CMakeFiles/RecastDemo.dir/Source/ConvexVolumeTool.cpp.o

In file included from /Users/grahampentheny/workspace/recastnavigation/RecastDemo/Source/ConvexVolumeTool.cpp:24:

/Library/Frameworks/SDL2.framework/Headers/SDL.h:32:10: fatal error: 'SDL2/SDL_main.h' file not found

#include <SDL2/SDL_main.h>

         ^~~~~~~~~~~~~~~~~

1 error generated.

make[2]: *** [RecastDemo/CMakeFiles/RecastDemo.dir/Source/ConvexVolumeTool.cpp.o] Error 1

make[1]: *** [RecastDemo/CMakeFiles/RecastDemo.dir/all] Error 2

make: *** [all] Error 2

```



I suspect this line in `RecastDemo/CMakeLists.txt` is to blame, though I'm not sure how frameworks are handled by cmake.

```

include_directories(Contrib/SDL/include)

```



---



## Fix Requirements:

We ultimately need the cmake script to invoke clang with the `-framework SDL2` and `-I/Library/Frameworks/SDL2.framework/Headers` options.  



It would also be nice to be able to specify the SDL framework path (via `-F` clang flag) so it's not just hardcoded to `/Library/Frameworks`.  Specifying the framework path like this is useful for CI processes that might not want to or be able to muck about in `/Library`.

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