Using exit() requires stdlib.h, which is not included.  Use return
instead.  Also add return type for main().
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Thomas Munro <thomas.munro@enterprisedb.com>
Backpatched because Apple macOS 10.16/11 (Big Sur) compiler makes
calling undeclared functions an error, so these configure tests would
fail.
Reported-by: Thomas Gilligan <thomas.gilligan@icloud.com>
Reported-by: Jesse Zhang <sbjesse@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/
09A4B554-82B1-4536-B191-
2461342EE0BB%40icloud.com
     return 0;
   return 1;
 }
+
+int
 main() {
-  exit(! does_int64_work());
+  return (! does_int64_work());
 }])],
 [Ac_cachevar=yes],
 [Ac_cachevar=no],
 
     return 0;          /* either multiply or snprintf is busted */
   return 1;
 }
+
+int
 main() {
-  exit(! does_int64_snprintf_work());
+  return (! does_int64_snprintf_work());
 }]])],
 [pgac_cv_snprintf_long_long_int_modifier=$pgac_modifier; break],
 [],
 
     return 0;
   return 1;
 }
+
+int
 main() {
-  exit(! does_int64_work());
+  return (! does_int64_work());
 }
 _ACEOF
 if ac_fn_c_try_run "$LINENO"; then :
     return 0;
   return 1;
 }
+
+int
 main() {
-  exit(! does_int64_work());
+  return (! does_int64_work());
 }
 _ACEOF
 if ac_fn_c_try_run "$LINENO"; then :
     return 0;          /* either multiply or snprintf is busted */
   return 1;
 }
+
+int
 main() {
-  exit(! does_int64_snprintf_work());
+  return (! does_int64_snprintf_work());
 }
 _ACEOF
 if ac_fn_c_try_run "$LINENO"; then :