Move `posix_signal` declaration internal with prefix `ruby_`

include/ruby/internal/intern/signal.h

@@ -113,12 +113,6 @@ RBIMPL_ATTR_NONNULL(())
  */
 VALUE rb_f_kill(int argc, const VALUE *argv);
 
-/* This must be private, @shyouhei guesses. */
-#ifdef POSIX_SIGNAL
-#define posix_signal ruby_posix_signal
-void (*posix_signal(int, void (*)(int)))(int);
-#endif
-
 RBIMPL_ATTR_PURE()
 /**
  * Queries  the name  of  the signal.   It returns  for  instance `"KILL"`  for

internal/signal.h

@@ -13,6 +13,10 @@
 extern int ruby_enable_coredump;
 int rb_get_next_signal(void);
 
+#ifdef POSIX_SIGNAL
+void (*ruby_posix_signal(int, void (*)(int)))(int);
+#endif
+
 RUBY_SYMBOL_EXPORT_BEGIN
 /* signal.c (export) */
 int rb_grantpt(int fd);

signal.c

@@ -628,7 +628,7 @@ ruby_signal(int signum, sighandler_t handler)
 }
 
 sighandler_t
-posix_signal(int signum, sighandler_t handler)
+ruby_posix_signal(int signum, sighandler_t handler)
 {
     return ruby_signal(signum, handler);
 }

thread_pthread.c

@@ -737,7 +737,7 @@ Init_native_thread(rb_thread_t *main_th)
         rb_bug("pthread_key_create failed (ruby_current_ec_key)");
     }
 #endif
-    posix_signal(SIGVTALRM, null_func);
+    ruby_posix_signal(SIGVTALRM, null_func);
 
     // setup main thread
     main_th->nt->thread_id = pthread_self();