examples: beware of portability issues with sh's trap

On AIX 7.2, when invoking "exit 77", we actually exit with 127.  The
"cleanup" function, called via trap, received an incorrect exit
status, something described in Autoconf's doc.
Reported by Bruno Haible.
https://lists.gnu.org/archive/html/bug-bison/2020-05/msg00029.html
https://lists.gnu.org/archive/html/bug-bison/2020-05/msg00047.html

* examples/test (skip): New.
* examples/c/bistromathic/bistromathic.test,
* examples/c/reccalc/reccalc.test: Use it, to ensure $? is set to 77
when the trap is called.

examples/c/bistromathic/bistromathic.test

@@ -46,8 +46,7 @@ if diff perfect effective >/dev/null 2>&1; then
 elif diff ok effective >/dev/null 2>&1; then
     strip_prompt=true
 else
-    echo "SKIP: this is not the GNU Readline we expect"
+    skip "this is not the GNU Readline we expect"
-    exit 77
 fi
 
 

examples/c/reccalc/reccalc.test

@@ -15,7 +15,7 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-(seq 0) >/dev/null 2>&1 || exit 77
+(seq 0) >/dev/null 2>&1 || skip "gimme one seq"
 
 cat >input <<EOF
 1+2*3

examples/test

@@ -68,6 +68,20 @@ trap cleanup 0 1 2 13 15
 mkdir $$.dir
 cd $$.dir
 
+
+# skip [MSG]
+# ----------
+# Skip this test.
+skip ()
+{
+  if test x"$1" != x; then
+    echo "SKIP: $1"
+  fi
+  # See Autoconf's doc on 'trap'.
+  (exit 77); exit 77
+}
+
+
 # run [-noerr, -n] EXPECTED-EXIT-STATUS EXPECTED-OUTPUT [PARSER-OPTIONS]
 # ----------------------------------------------------------------------
 # -noerr: ignore stderr, otherwise merge it into effective output.