Disable warnings in system headers in MSVC

Disable warnings in system headers in MSVC

This is similar to the standard behavior in GCC.  For MSVC, we set all
headers in angle brackets to be considered system headers.  (GCC goes
by path, not include style.)

The required option is available since VS 2017.  (Before VS 2019
version 16.10, the additional option /experimental:external is
required, but per discussion in [0], we effectively require 16.11, so
this shouldn't be a problem.)

[0]: https://www.postgresql.org/message-id/04ab76a3-186c-4a37-8076-e6882ebf9d43%40eisentraut.org

Then, we can remove one workaround for avoiding a warning from a
system header.  (And some warnings to be enabled in the future could
benefit from this.)

Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/aa73q1aT0A3/vke/%40ip-10-97-1-34.eu-west-3.compute.internal