pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2f7bae2f924d8213a76370f825dc15eb0aa79796

Modified Files
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src/interfaces/libpq/.gitignore | 1 +
src/interfaces/libpq/Makefile | 10 +++++-----
2 files changed, 6 insertions(+), 5 deletions(-)