build: stop using an obsolete macro, and use 'void' for signal handlers

The Autoconf manual says that 'AC_TYPE_SIGNAL' is obsolete and
that it is fine to simply use 'void' instead of 'RETSIGTYPE'.

configure.ac

@@ -543,10 +543,6 @@ dnl Checks for available flags.
 
 AX_CHECK_COMPILE_FLAG([-Wall], [CFLAGS="$CFLAGS -Wall"], [], [])
 
-dnl Checks for library functions.
-
-AC_TYPE_SIGNAL
-
 dnl Checks for libraries.
 
 if eval "test x$CURSES_LIB_NAME = x"; then

src/files.c

@@ -916,7 +916,7 @@ static pid_t pid_of_command = -1;
 		/* The PID of a forked process -- needed when wanting to abort it. */
 
 /* Send an unconditional kill signal to the running external command. */
-RETSIGTYPE cancel_the_command(int signal)
+void cancel_the_command(int signal)
 {
 	kill(pid_of_command, SIGKILL);
 }

src/nano.c

@@ -780,7 +780,7 @@ void version(void)
 }
 
 /* Register that Ctrl+C was pressed during some system call. */
-RETSIGTYPE make_a_note(int signal)
+void make_a_note(int signal)
 {
 	control_C_was_pressed = TRUE;
 }
@@ -915,21 +915,21 @@ void signal_init(void)
 }
 
 /* Handler for SIGHUP (hangup) and SIGTERM (terminate). */
-RETSIGTYPE handle_hupterm(int signal)
+void handle_hupterm(int signal)
 {
 	die(_("Received SIGHUP or SIGTERM\n"));
 }
 
 #if !defined(NANO_TINY) && !defined(DEBUG)
 /* Handler for SIGSEGV (segfault) and SIGABRT (abort). */
-RETSIGTYPE handle_crash(int signal)
+void handle_crash(int signal)
 {
 	die(_("Sorry! Nano crashed!  Code: %d.  Please report a bug.\n"), signal);
 }
 #endif
 
 /* Handler for SIGTSTP (suspend). */
-RETSIGTYPE do_suspend(int signal)
+void do_suspend(int signal)
 {
 #ifdef ENABLE_MOUSE
 	disable_mouse_support();
@@ -971,7 +971,7 @@ void do_suspend_void(void)
 }
 
 /* Handler for SIGCONT (continue after suspend). */
-RETSIGTYPE do_continue(int signal)
+void do_continue(int signal)
 {
 #ifdef ENABLE_MOUSE
 	if (ISSET(USE_MOUSE))
@@ -1013,7 +1013,7 @@ void block_sigwinch(bool blockit)
 
 #ifndef NANO_TINY
 /* Handler for SIGWINCH (window size change). */
-RETSIGTYPE handle_sigwinch(int signal)
+void handle_sigwinch(int signal)
 {
 	/* Let the input routine know that a SIGWINCH has occurred. */
 	the_window_resized = TRUE;

src/prototypes.h

@@ -403,17 +403,17 @@ void restore_handler_for_Ctrl_C(void);
 #ifndef NANO_TINY
 void reconnect_and_store_state(void);
 #endif
-RETSIGTYPE handle_hupterm(int signal);
+void handle_hupterm(int signal);
 #ifndef DEBUG
-RETSIGTYPE handle_crash(int signal);
+void handle_crash(int signal);
 #endif
-RETSIGTYPE do_suspend(int signal);
+void do_suspend(int signal);
-RETSIGTYPE do_continue(int signal);
+void do_continue(int signal);
 #if !defined(NANO_TINY) || defined(ENABLE_SPELLER) || defined(ENABLE_COLOR)
 void block_sigwinch(bool blockit);
 #endif
 #ifndef NANO_TINY
-RETSIGTYPE handle_sigwinch(int signal);
+void handle_sigwinch(int signal);
 void compute_the_extra_rows_per_line_from(linestruct *fromline);
 void regenerate_screen(void);
 void do_toggle(int flag);