maint: detect unnecessary inclusion of signal.h

* maint.mk (sc_prohibit_signal_without_use): New rule.

maint.mk

@@ -289,6 +289,43 @@ ctype_re = isalnum|isalpha|isascii|isblank|iscntrl|isdigit|isgraph|islower\
 sc_prohibit_c_ctype_without_use:
 	@h='[<"]c-ctype.h[">]' re='\<c_($(ctype_re)) *\(' $(_header_without_use)
 
+_empty =
+_sp = $(_empty) $(_empty)
+# The following list was generated by running:
+# man signal.h|col -b|perl -ne '/bsd_signal.*;/.../sigwaitinfo.*;/ and print' \
+#   | perl -lne '/^\s+(?:int|void).*?(\w+).*/ and print $1' | fmt
+_sig_functions = \
+  bsd_signal kill killpg pthread_kill pthread_sigmask raise sigaction \
+  sigaddset sigaltstack sigdelset sigemptyset sigfillset sighold sigignore \
+  siginterrupt sigismember signal sigpause sigpending sigprocmask sigqueue \
+  sigrelse sigset sigsuspend sigtimedwait sigwait sigwaitinfo
+_sig_function_re = $(subst $(_sp),|,$(strip $(_sig_functions)))
+# The following were extracted from "man signal.h" manually.
+_sig_types_and_consts =							\
+  MINSIGSTKSZ SA_NOCLDSTOP SA_NOCLDWAIT SA_NODEFER SA_ONSTACK		\
+  SA_RESETHAND SA_RESTART SA_SIGINFO SIGEV_NONE SIGEV_SIGNAL		\
+  SIGEV_THREAD SIGSTKSZ SIG_BLOCK SIG_SETMASK SIG_UNBLOCK SS_DISABLE	\
+  SS_ONSTACK mcontext_t pid_t sig_atomic_t sigevent siginfo_t sigset_t	\
+  sigstack sigval stack_t ucontext_t
+# generated via this:
+# perl -lne '/^#ifdef (SIG\w+)/ and print $1' lib/sig2str.c|sort -u|fmt -70
+_sig_names =								\
+  SIGABRT SIGALRM SIGALRM1 SIGBUS SIGCANCEL SIGCHLD SIGCLD SIGCONT	\
+  SIGDANGER SIGDIL SIGEMT SIGFPE SIGFREEZE SIGGRANT SIGHUP SIGILL	\
+  SIGINFO SIGINT SIGIO SIGIOT SIGKAP SIGKILL SIGKILLTHR SIGLOST SIGLWP	\
+  SIGMIGRATE SIGMSG SIGPHONE SIGPIPE SIGPOLL SIGPRE SIGPROF SIGPWR	\
+  SIGQUIT SIGRETRACT SIGSAK SIGSEGV SIGSOUND SIGSTKFLT SIGSTOP SIGSYS	\
+  SIGTERM SIGTHAW SIGTRAP SIGTSTP SIGTTIN SIGTTOU SIGURG SIGUSR1	\
+  SIGUSR2 SIGVIRT SIGVTALRM SIGWAITING SIGWINCH SIGWIND SIGWINDOW	\
+  SIGXCPU SIGXFSZ
+_sig_syms_re = $(subst $(_sp),|,$(strip $(_sig_names) $(_sig_types_and_consts)))
+
+# Prohibit the inclusion of signal.h without an actual use.
+sc_prohibit_signal_without_use:
+	@h='<signal.h>'							\
+	re='\<($(_sig_function_re)) *\(|\<($(_sig_syms_re))\>'		\
+	  $(_header_without_use)
+
 sc_obsolete_symbols:
 	@re='\<(HAVE''_FCNTL_H|O''_NDELAY)\>'				\
 	msg='do not use HAVE''_FCNTL_H or O'_NDELAY			\