pgsql: Try another way to detect the result type of strerror_r().

Try another way to detect the result type of strerror_r().

The method we've traditionally used, of redeclaring strerror_r() to
see if the compiler complains of inconsistent declarations, turns out
not to work reliably because some compilers only report a warning,
not an error. Amazingly, this has gone undetected for years, even
though it certainly breaks our detection of whether strerror_r
succeeded.

Let's instead test whether the compiler will take the result of
strerror_r() as a switch() argument. It's possible this won't
work universally either, but it's the best idea I could come up with
on the spur of the moment.

We should probably back-patch this once the dust settles, but
first let's see what the buildfarm thinks of it.

Discussion: https://postgr.es/m/10877.1537993279@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/751f532b9766fb5d3c334758abea95b7bb085c5a

Modified Files
--------------
config/c-library.m4 | 18 +++++++++---------
configure | 10 ++++------
src/include/pg_config.h.in | 2 +-
src/include/pg_config.h.win32 | 2 +-
4 files changed, 15 insertions(+), 17 deletions(-)