pgsql: oauth: Fix build on platforms without epoll/kqueue

oauth: Fix build on platforms without epoll/kqueue

register_socket() missed a variable declaration if neither
HAVE_SYS_EPOLL_H nor HAVE_SYS_EVENT_H was defined.

While we're fixing that, adjust the tests to check pg_config.h for one
of the multiplexer implementations, rather than assuming that Windows is
the only platform without support. (Christoph reported this on
hurd-amd64, an experimental Debian.)

Author: Jacob Champion <jacob(dot)champion(at)enterprisedb(dot)com>
Reported-by: Christoph Berg <myon(at)debian(dot)org>
Discussion: https://postgr.es/m/Z-sPFl27Y0ZC-VBl%40msg.df7cb.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8ae0a37932d81082ec7e4b10eb304a83c561408e

Modified Files
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src/interfaces/libpq/fe-auth-oauth-curl.c | 4 ++--
src/test/modules/oauth_validator/t/001_server.pl | 6 ++++--
2 files changed, 6 insertions(+), 4 deletions(-)