Explicitly set -std=gnu99 (via CMAKE_C_STANDARD) for linux targets when using the bundled feature.

[Grinkers]

https://lwn.net/ml/all/aAtlfgZQyPsfBPKA@tucnak/
The C frontend now defaults to the GNU C23 dialect.

With the latest GCC, when using bundled and static-link features, gcc tries to compile with c23 which results in a build failure due to SDL2 using reserved keywords here
https://github.com/libsdl-org/SDL/blob/07d0f51fa292895443f563f0cbde4cb3802d87fa/src/joystick/hidapi/SDL_hidapi_steam.c#L40-L44

Here's the error log for cargo build --features bundled,static-link

  /home/grinkers/code/rust-sdl2/sdl2-sys/SDL/src/joystick/hidapi/SDL_hidapi_steam.c:42:5: error: cannot use keyword 'false' as enumeration constant
     42 |     false,
        |     ^~~~~
  /home/grinkers/code/rust-sdl2/sdl2-sys/SDL/src/joystick/hidapi/SDL_hidapi_steam.c:42:5: note: 'false' is a keyword with '-std=c23' onwards
  /home/grinkers/code/rust-sdl2/sdl2-sys/SDL/src/joystick/hidapi/SDL_hidapi_steam.c:44:3: error: expected ';', identifier or '(' before 'bool'
     44 | } bool;
        |   ^~~~
  /home/grinkers/code/rust-sdl2/sdl2-sys/SDL/src/joystick/hidapi/SDL_hidapi_steam.c:44:3: warning: useless type name in empty declaration
  gmake[2]: *** [CMakeFiles/SDL2-static.dir/build.make:3067: CMakeFiles/SDL2-static.dir/src/joystick/hidapi/SDL_hidapi_steam.c.o] Error 1
  gmake[2]: *** Waiting for unfinished jobs....
  gmake[1]: *** [CMakeFiles/Makefile2:159: CMakeFiles/SDL2-static.dir/all] Error 2
  gmake: *** [Makefile:136: all] Error 2

  thread 'main' panicked at /home/grinkers/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/cmake-0.1.54/src/lib.rs:1119:5:

  command did not execute successfully, got: exit status: 2

  build script failed, must exit now

[Grinkers]

Original PR was -std=gnu17, but I realized this could be a very breaking change for people on old systems. I changed it to gnu99.

https://github.com/Grinkers/rust-sdl2/actions/runs/14670059227/job/41174276590
It seems to be working fine on at least CI and gcc15.