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.hpp

@@ -0,0 +1,21 @@
+/*
+ * Copyright OpenEmbedded Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+#include <string>
+
+struct CppExample
+{
+    inline static const std::string test_string = "cpp-example-lib Magic: 123456789";
+
+    /* Retrieve a constant string */
+    const std::string &get_string();
+    /* Retrieve a constant string from a library */
+    const char *get_json_c_version();
+    /* Call a more advanced function from a library */
+    void print_json();
+};