fix the build on OpenBSD

[guijan]

Hi, the build fails on OpenBSD because the X11 include directory is missing. This small change fixes it.

The FindX11.cmake module seems to only trigger on Unix so I'm not putting it in a conditional or anything: https://github.com/Kitware/CMake/blob/3585d117e8da6e3423d515616b3fcc4869cbd817/Modules/FindX11.cmake#L93

[k4zmu2a]

Hello.

This fix looks fine to me, but first I would like to you to try an alternative solution.
Add SDL defines that hide X11 includes from SDL headers, should be safe to do since the game does not use SDL_SYSWMEVENT or EGL.

Pch.h:
#define SDL_MAIN_HANDLED
#define SDL_PROTOTYPES_ONLY
#define MESA_EGL_NO_X11_HEADERS
#include "SDL.h"
#include <SDL_mixer.h>

[guijan]

Still fails with that fix:

$ cmake --build build --config Release                              
[8/69] Building CXX object CMakeFiles/SpaceCad...all.dir/SpaceCadetPinball/imgui_impl_sdl.cpp.o
FAILED: CMakeFiles/SpaceCadetPinball.dir/SpaceCadetPinball/imgui_impl_sdl.cpp.o 
/usr/bin/c++  -I/usr/local/include/SDL2 -std=gnu++11 -MD -MT CMakeFiles/SpaceCadetPinball.dir/SpaceCadetPinball/imgui_impl_sdl.cpp.o -MF CMakeFiles/SpaceCadetPinball.dir/SpaceCadetPinball/imgui_impl_sdl.cpp.o.d -o CMakeFiles/SpaceCadetPinball.dir/SpaceCadetPinball/imgui_impl_sdl.cpp.o -c ../SpaceCadetPinball/imgui_impl_sdl.cpp
In file included from ../SpaceCadetPinball/imgui_impl_sdl.cpp:58:
/usr/local/include/SDL2/SDL_syswm.h:69:10: fatal error: 'X11/Xlib.h' file not found
#include <X11/Xlib.h>
         ^~~~~~~~~~~~
1 error generated.
[13/69] Building CXX object CMakeFiles/SpaceCadetPinball.dir/SpaceCadetPinball/control.cpp.o
ninja: build stopped: subcommand failed.

[k4zmu2a]

Ok, so the source of X11 include leak is not SDL.h in pch.
As it turns out, there is a direct include of SDL_syswm.h in ImGui, and it is only needed on win32.

Disregard the previous fix; try this next, in imgui_impl_sdl.cpp:
// SDL
#include <SDL.h>
#ifdef _WIN32
#include <SDL_syswm.h>
#endif

[guijan]

That change does fix the build on OpenBSD, I've updated the PR with it.

[k4zmu2a]

Merged.
It is good that it works without unnecessary CMake package dependency.