testthread isn't thread-safe: non-locked access to global variable 'alive'

[smcv]

In testthread, the main thread and a helper thread communicate via a global variable alive, but concurrent non-atomic accesses to variables are not thread-safe (undefined behaviour). ThreadSanitizer diagnoses this:

WARNING: ThreadSanitizer: data race (pid=1486312)
  Read of size 4 at 0x55c924964494 by thread T1:
    #0 ThreadFunc /home/smcv/src/SDL/test/testthread.c:65 (testthread+0x14df8) (BuildId: ff8455198b6ec3b4e9d2d60887c1c34f84865e83)
    #1 SDL_RunThread /home/smcv/src/SDL/src/thread/SDL_thread.c:323 (libSDL3.so.0+0x850252) (BuildId: 2ad90d76e3ecc027c5bab3986c6c2f3310e6312f)
    #2 RunThread /home/smcv/src/SDL/src/thread/pthread/SDL_systhread.c:69 (libSDL3.so.0+0xc89efd) (BuildId: 2ad90d76e3ecc027c5bab3986c6c2f3310e6312f)

  Previous write of size 4 at 0x55c924964494 by main thread:
    #0 main /home/smcv/src/SDL/test/testthread.c:147 (testthread+0x151f6) (BuildId: ff8455198b6ec3b4e9d2d60887c1c34f84865e83)

  As if synchronized via sleep:
    #0 nanosleep ../../../../src/libsanitizer/tsan/tsan_interceptors_posix.cpp:397 (libtsan.so.2+0x54d2f) (BuildId: 10df5f7ccae7f4e0cda410a7307bbff5fb61b777)
    #1 SDL_SYS_DelayNS /home/smcv/src/SDL/src/timer/unix/SDL_systimer.c:170 (libSDL3.so.0+0xc92065) (BuildId: 2ad90d76e3ecc027c5bab3986c6c2f3310e6312f)
    #2 SDL_Delay_REAL /home/smcv/src/SDL/src/timer/SDL_timer.c:638 (libSDL3.so.0+0x853caf) (BuildId: 2ad90d76e3ecc027c5bab3986c6c2f3310e6312f)
    #3 SDL_Delay /home/smcv/src/SDL/src/dynapi/SDL_dynapi_procs.h:165 (libSDL3.so.0+0x622d35) (BuildId: 2ad90d76e3ecc027c5bab3986c6c2f3310e6312f)
    #4 ThreadFunc /home/smcv/src/SDL/test/testthread.c:75 (testthread+0x14dec) (BuildId: ff8455198b6ec3b4e9d2d60887c1c34f84865e83)
    #5 SDL_RunThread /home/smcv/src/SDL/src/thread/SDL_thread.c:323 (libSDL3.so.0+0x850252) (BuildId: 2ad90d76e3ecc027c5bab3986c6c2f3310e6312f)
    #6 RunThread /home/smcv/src/SDL/src/thread/pthread/SDL_systhread.c:69 (libSDL3.so.0+0xc89efd) (BuildId: 2ad90d76e3ecc027c5bab3986c6c2f3310e6312f)

  Location is global 'alive' of size 4 at 0x55c924964494 (testthread+0x41494)

  Thread T1 'One' (tid=1486314, running) created by main thread at:
    #0 pthread_create ../../../../src/libsanitizer/tsan/tsan_interceptors_posix.cpp:1022 (libtsan.so.2+0x568a6) (BuildId: 10df5f7ccae7f4e0cda410a7307bbff5fb61b777)
    #1 SDL_SYS_CreateThread /home/smcv/src/SDL/src/thread/pthread/SDL_systhread.c:111 (libSDL3.so.0+0xc89fca) (BuildId: 2ad90d76e3ecc027c5bab3986c6c2f3310e6312f)
    #2 SDL_CreateThreadWithPropertiesRuntime_REAL /home/smcv/src/SDL/src/thread/SDL_thread.c:383 (libSDL3.so.0+0x8505dc) (BuildId: 2ad90d76e3ecc027c5bab3986c6c2f3310e6312f)
    #3 SDL_CreateThreadRuntime_REAL /home/smcv/src/SDL/src/thread/SDL_thread.c:403 (libSDL3.so.0+0x8507c7) (BuildId: 2ad90d76e3ecc027c5bab3986c6c2f3310e6312f)
    #4 SDL_CreateThreadRuntime /home/smcv/src/SDL/src/dynapi/SDL_dynapi_procs.h:158 (libSDL3.so.0+0x622b43) (BuildId: 2ad90d76e3ecc027c5bab3986c6c2f3310e6312f)
    #5 main /home/smcv/src/SDL/test/testthread.c:140 (testthread+0x151ad) (BuildId: ff8455198b6ec3b4e9d2d60887c1c34f84865e83)

SUMMARY: ThreadSanitizer: data race /home/smcv/src/SDL/test/testthread.c:65 in ThreadFunc

This should probably be a SDL_AtomicInt?