cmake: add C++ integration test

Also:
- include code to verify a C++-specific public header regression
  reported in 8.19.0-rc2.
- curl/curl.h: mention C++ global namespace in comment.
- GHA/dist: add CI job for C++. Runtime: 15 seconds.

Follow-up to ee9b000438 #20686
Ref: #20682

Closes #20687

tests/cmake/test.cpp

@@ -0,0 +1,44 @@
+/***************************************************************************
+ *                                  _   _ ____  _
+ *  Project                     ___| | | |  _ \| |
+ *                             / __| | | | |_) | |
+ *                            | (__| |_| |  _ <| |___
+ *                             \___|\___/|_| \_\_____|
+ *
+ * Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+ *
+ * This software is licensed as described in the file COPYING, which
+ * you should have received as part of this distribution. The terms
+ * are also available at https://curl.se/docs/copyright.html.
+ *
+ * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+ * copies of the Software, and permit persons to whom the Software is
+ * furnished to do so, under the terms of the COPYING file.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ * SPDX-License-Identifier: curl
+ *
+ ***************************************************************************/
+#include <curl/curl.h>
+#include <iostream>
+
+class CurlClass {
+public:
+  void curl_multi_setopt(void *a, int b, long c) {
+    std::cout << curl_version() << std::endl;
+  }
+};
+
+int main(int argc, const char **argv)
+{
+  (void)argc;
+  std::cout << "libcurl C++ test:" << std::endl;
+  std::cout << argv[0] << std::endl;
+  CurlClass mycurl;
+  mycurl.curl_multi_setopt(nullptr, 0, 0);
+  ::curl_easy_setopt(nullptr, CURLOPT_URL, "https://example.com/");
+  std::cout << "---" << std::endl;
+  return 0;
+}