pgsql: Fix configure's snprintf test so it exposes HP-UX bug.

Fix configure's snprintf test so it exposes HP-UX bug.

Since commit e1d19c902, buildfarm member gharial has been failing with
symptoms indicating that snprintf sometimes returns -1 for buffer
overrun, even though it passes the added configure check.  Some
google research suggests that this happens only in limited cases,
such as when the overrun happens partway through a %d item.  Adjust
the configure check to exercise it that way.  Since I'm now feeling
more paranoid than I was before, also make the test explicitly verify
that the buffer doesn't get physically overrun.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9bed827b18cc4f27fb7cd7c02ad301519eca6d29

Modified Files
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config/c-library.m4 | 11 ++++++++---
configure           |  9 ++++++---
2 files changed, 14 insertions(+), 6 deletions(-)