Use return instead of exit() in configure

Use return instead of exit() in configure

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