Fix SDL threading on macOS/Linux.

[bruvzg]

These files are used only on Windows and seems to cause infinite loop on macOS.

Bugsquad edit:

• Fixup to bugs introduced in Add support for SDL3 joystick input driver for Windows, Linux and macOS #106218 at the last minute.

[akien-mga]

This was likely the main issue, it seems like I saw it included in the Windows driver:

thirdparty/sdl/thread/windows/SDL_systhread.c
29:#include "SDL_systhread_c.h"

But I failed to realize this is thread/windows/SDL_systhread_c.h and not thread/generic/SDL_systhread.c.

It's super confusing that they have files with the same names in different folders (so they seem mutually exclusive) but then the Windows one depends on a couple thread/generic files anyway...

[akien-mga]

I also fixed the SDL update script to take this into account and document it better, and add the pkgconfig Linux fixup for #106218 (comment).

[akien-mga]

lol what a mess:

Error: thirdparty/sdl/thread/pthread/../generic/SDL_syssem.c:25:10: fatal error: 'SDL_systhread_c.h' file not found
#include "SDL_systhread_c.h"
scons: building terminated because of errors.
         ^~~~~~~~~~~~~~~~~~~

So we shouldn't build thread/generic/SDL_systhread.c as that breaks everything, but thread/generic/SDL_systhread_c.h is needed for thread/generic/SDL_syssem.c which is included automatically for macOS.

And same issue for the Windows part:

Error: thirdparty\sdl\thread\generic\SDL_sysrwlock.c(25): fatal error C1083: Cannot open include file: 'SDL_systhread_c.h': No such file or directory
scons: *** [thirdparty\sdl\thread\generic\SDL_sysrwlock.windows.editor.x86_64.obj] Error 2

So I guess I'll add back the header but without the corresponding C file to avoid the risk of compiling it... this could definitely be improved somewhat upstream to be more self-contained.

[akien-mga]

Seems to work fine on Linux, tested with an Xbox Series controller on bluetooth.

[akien-mga]

Thanks!