Improve build-time check that libpq doesn't call exit().

 Improve build-time check that libpq doesn't call exit().
 
 Further fixes for commit dc227eb82.  Per suggestion from
 Peter Eisentraut, use a stamp-file to control when the check
 is run, avoiding repeated executions during "make all".
 Also, remove "-g" switch for nm: it's useless and some versions
 of nm consider it to conflict with "-u".  (Thanks to Noah Misch
 for running down that portability issue.)
 
 Discussion: https://postgr.es/m/3128896.1624742969@sss.pgh.pa.us