oe-selftest: add a cpp-example recipe

This simple C++ project supports compilation with CMake and Meson.
(Autotool support could be added later on.)
It's supposed to be used with oe-selftest.

An artificial project has several advantages over compiling a normal
CMake or Meson based project for testing purposes:
- It is much faster because it can be kept minimalistic
- It can cover multiple odd corner cases
- No one will change it in an unpredictable way
- It can support multiple build tools with only one C++ codebase

(From OE-Core rev: 4904e772470b0d6e5d98ef0344b3f2bf54214661)

Signed-off-by: Adrian Freihofer <adrian.freihofer@siemens.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta-selftest/recipes-test/cpp/files/cpp-example-lib.cpp

@@ -0,0 +1,33 @@
+/*
+ * Copyright OpenEmbedded Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <iostream>
+#include <string>
+#include <json-c/json.h>
+#include "cpp-example-lib.hpp"
+
+const std::string &CppExample::get_string()
+{
+    return test_string;
+}
+
+const char *CppExample::get_json_c_version()
+{
+    return json_c_version();
+}
+
+void CppExample::print_json()
+{
+    struct json_object *jobj;
+    const int flag = JSON_C_TO_STRING_SPACED | JSON_C_TO_STRING_PRETTY;
+
+    jobj = json_object_new_object();
+    json_object_object_add(jobj, "test_string", json_object_new_string(test_string.c_str()));
+
+    std::cout << json_object_to_json_string_ext(jobj, flag) << std::endl;
+
+    json_object_put(jobj); // Delete the json object
+}