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
------
master

Details
-------
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(-)