generic glibc: guard pidfd_spawn(p) declarations

lib/libc/include/generic-glibc/bits/spawn_ext.h

@@ -35,6 +35,8 @@ extern int posix_spawnattr_setcgroup_np (posix_spawnattr_t *__attr,
 					 int __cgroup)
      __THROW __nonnull ((1));
 
+// zig patch: check target glibc version
+#if (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 39) || __GLIBC__ > 2
 /* Spawn a new process executing PATH with the attributes describes in *ATTRP.
    Before running the process perform the actions described in FACTS.  Return
    a PID file descriptor in PIDFD if process creation was successful and the
@@ -61,7 +63,8 @@ extern int pidfd_spawnp (int *__restrict __pidfd,
 			 char *const __argv[__restrict_arr],
 			 char *const __envp[__restrict_arr])
     __nonnull ((1, 2, 5));
+#endif /* (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 39) || __GLIBC__ > 2 */
 
 #endif /* __USE_MISC */
 
-__END_DECLS
+__END_DECLS