BUILD: time: remove the test on _POSIX_C_SOURCE

It seems it's not defined on FreeBSD while it's mentioned on Linux that
clock_gettime() can be detected using this. Given that we also have the
test for _POSIX_TIMERS>0 that should cover it well enough. If it breaks
on other systems, we'll see.

Report was here :
    https://github.com/haproxy/haproxy/runs/133866993

include/common/compat.h

@@ -105,7 +105,7 @@ typedef struct { } empty_t;
 #endif
 
 /* systems without such defines do not know clockid_t or timer_t */
-#if !(_POSIX_TIMERS > 0) || (_POSIX_C_SOURCE < 199309L)
+#if !(_POSIX_TIMERS > 0)
 #undef clockid_t
 #define clockid_t empty_t
 #undef timer_t

src/wdt.c

@@ -25,7 +25,7 @@
  * It relies on timer_create() and timer_settime() which are only available in
  * this case.
  */
-#if defined(USE_THREAD) && (_POSIX_TIMERS > 0) && defined(_POSIX_THREAD_CPUTIME) && (_POSIX_C_SOURCE >= 199309L)
+#if defined(USE_THREAD) && (_POSIX_TIMERS > 0) && defined(_POSIX_THREAD_CPUTIME)
 
 /* We'll deliver SIGALRM when we've run out of CPU as it's not intercepted by
  * gdb by default.