tests: minor improvement of sc_tight_scope rule

* src/Makefile.am (sc_tight_scope): Adjust rule to use an eval-based
trap-setting for-loop rather than 4x hard-coded 128+N constants.
Also catch SIGQUIT (3).  Tweak comments.

src/Makefile.am

@@ -707,8 +707,8 @@ sc_check-AUTHORS: $(all_programs)
 
 # Most functions in src/*.c should have static scope.
 # Any that don't must be marked with `extern', but `main'
-# and `usage' are exceptions.  They're always extern, but
-# don't need to be marked. Also functions starting with __
+# and `usage' are exceptions: they're always extern, but
+# do not need to be marked.  Also functions starting with __
 # are exempted due to possibly being added by the compiler
 # (when compiled as a shared library for example).
 #
@@ -717,9 +717,9 @@ sc_check-AUTHORS: $(all_programs)
 sc_tight_scope: $(bin_PROGRAMS)
 	@t=exceptions-$$$$;						\
 	trap 's=$$?; rm -f $$t; exit $$s' 0;				\
-	am__exit='(exit $s); exit $s';					\
-	trap "s=129; $$am__exit" 1; trap "s=130; $$am__exit" 2;		\
-	trap "s=141; $$am__exit" 13; trap "s=143; $$am__exit" 15;	\
+	for sig in 1 2 3 13 15; do					\
+	  eval "trap 'v=`expr $$sig + 128`; (exit $$v); exit $$v' $$sig"; \
+	done;								\
 	src=`for f in $(SOURCES); do					\
 	       test -f $$f && d= || d=$(srcdir)/; echo $$d$$f; done`;	\
 	hdr=`for f in $(noinst_HEADERS); do				\