Here's a small but representative example.

#if HAVE_SYS_TIME_H

#include <sys/time.h

#elif HAVE_TIME_H

#include <time.h>

#endif

#if HAVE_SYS_CLOCK_GETTIME

time_t

time_now(void)
{
  struct timespec timespec_value;

  (void) clock_gettime(CLOCK_REALTIME, &timespec_value);

  return timespec_value.tv_seconds;

}

#elif HAVE_SYS_GETTIMEOFDAY

time_t

time_now(void)
{
  struct timeval timeval_value;

  (void) gettimeofday(&timeval_value, (struct timezone *) NULL);

  return timeval_value.tv_seconds;

}

#elif HAVE_SYS_TIME

time_t

time_now(void)
{
  time_t seconds_since_epoch;

  (void) time(&seconds_since_epoch);

  return seconds_since_epoch;

}

#endif

./configure    # work out which HAVE_... definitions to use

Usually there are a few more alternatives enumerated.  Surprisingly often, the microseconds or nanoseconds

value is discarded, to get the seconds. You could just use #include <time.h> and call time(NULL) to get that, but where's the fun?