Define strerror() for Windows

The symbol `pgwin32_socket_strerror` was undefined on windows builds at
PG12 and later. This because the function was removed and instead
`pg_strerror` was introduced to be used on all platforms.

This commit fixes the issue by ensuring to use `pg_strerror` on PG12
and later, and `pgwin32_socket_strerror` on Windows builds before PG12.

Found using `clang-tidy`

src/net/conn_plain.c

@@ -11,6 +11,8 @@
 
 #include "conn_internal.h"
 #include "conn_plain.h"
+#include "compat.h"
+#include "port.h"
 
 #define DEFAULT_TIMEOUT_MSEC 3000
 #define MAX_PORT 65535
@@ -25,6 +27,24 @@ set_error(int err)
 #endif
 }
 
+static int
+get_error(void)
+{
+#ifdef WIN32
+	return WSAGetLastError();
+#else
+	return errno;
+#endif
+}
+
+/* We cannot define `pg_strerror` here because there is a #define in PG12 that
+ * sets `strerror` to `pg_strerror`. Instead, we handle the missing case for
+ * pre-PG12 on Windows by setting `strerror` to the windows version of the
+ * function and use `strerror` below. */
+#if PG12_LT && defined(WIN32)
+#define strerror(ERRNO) pgwin32_socket_strerror((ERRNO))
+#endif
+
 /*  Create socket and connect */
 int
 ts_plain_connect(Connection *conn, const char *host, const char *servname, int port)
@@ -227,14 +247,8 @@ ts_plain_errmsg(Connection *conn)
 {
 	const char *errmsg = "no connection error";
 
-#ifdef WIN32
 	if (IS_SOCKET_ERROR(conn->err))
-		errmsg = pgwin32_socket_strerror(WSAGetLastError());
-#else
-	if (IS_SOCKET_ERROR(conn->err))
-		errmsg = strerror(errno);
-#endif
-
+		errmsg = strerror(get_error());
 	conn->err = 0;
 
 	return errmsg;