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.champion@enterprisedb.com>
Reported-by: Christoph Berg <myon@debian.org>
Discussion: https://postgr.es/m/Z-sPFl27Y0ZC-VBl%40msg.df7cb.de

src/interfaces/libpq/fe-auth-oauth-curl.c

@@ -1172,8 +1172,9 @@ static int
 register_socket(CURL *curl, curl_socket_t socket, int what, void *ctx,
 				void *socketp)
 {
-#ifdef HAVE_SYS_EPOLL_H
 	struct async_ctx *actx = ctx;
+
+#ifdef HAVE_SYS_EPOLL_H
 	struct epoll_event ev = {0};
 	int			res;
 	int			op = EPOLL_CTL_ADD;
@@ -1231,7 +1232,6 @@ register_socket(CURL *curl, curl_socket_t socket, int what, void *ctx,
 	return 0;
 #endif
 #ifdef HAVE_SYS_EVENT_H
-	struct async_ctx *actx = ctx;
 	struct kevent ev[2] = {0};
 	struct kevent ev_out[2];
 	struct timespec timeout = {0};

src/test/modules/oauth_validator/t/001_server.pl

@@ -26,9 +26,11 @@ if (!$ENV{PG_TEST_EXTRA} || $ENV{PG_TEST_EXTRA} !~ /\boauth\b/)
 	  'Potentially unsafe test oauth not enabled in PG_TEST_EXTRA';
 }
 
-if ($windows_os)
+unless (check_pg_config("#define HAVE_SYS_EVENT_H 1")
+	or check_pg_config("#define HAVE_SYS_EPOLL_H 1"))
 {
-	plan skip_all => 'OAuth server-side tests are not supported on Windows';
+	plan skip_all =>
+	  'OAuth server-side tests are not supported on this platform';
 }
 
 if ($ENV{with_libcurl} ne 'yes')