How is Ada and GPR supposed to handle conditional compilation?

Question

I'm rewriting C source to Ada and on some places there is conditional compilation for handling different platforms, such as windows vs posix, DEBUG, or architecture. For what I can tell neither Ada nor GPR has the notion of conditional compilation.

What's the Ada way of handling it?

Example:

/* Determine if a process is alive. */
#ifndef _WIN32
   if (kill(pid, 0) && errno == ESRCH)
      return 0; /* pid does not exist */
#else
   HANDLE h;
   if ((h = OpenProcess(PROCESS_QUERY_INFORMATION, FALSE, (DWORD)pid)) == 0)
      return 0; /* pid does not exist */
   CloseHandle(h);
#endif
   return 1;