Use native system CA store when contacting website (#56)

This fixes the use of concordance on Windows where the bundled libcurl
can't by default find a valid CA store, resulting in failure to contact
the website.

Fixes #55.

libconcord/web.cpp

@@ -117,6 +117,9 @@ static int Zap(string &server, string &path, string &cookie, string &post)
 #else
         curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, dummy_cb);
 #endif
+#if LIBCURL_VERSION_NUM >= 0x074700
+        curl_easy_setopt(curl, CURLOPT_SSL_OPTIONS, CURLSSLOPT_NATIVE_CA);
+#endif
 
         CURLcode res = curl_easy_perform(curl);
         if (res != CURLE_OK) {