Zephyr: Support C++ builds

Some of the Zephyr include files need to be C++, not C based. The 'extern "C"'
wrapper in include/coap3/coap.h means that all #include are treated as being C.

Remove 'extern "C"' wrapper from include/coap3/coap.h and add wrapper to
include files after #include and before any function / structure definitions.

Add in a C++ build test for CI.

examples/zephyr/test-cpp-src/CMakeLists.txt

@@ -0,0 +1,9 @@
+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.20.0)
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(coap_client)
+
+target_sources(app PRIVATE src/test.cpp)
+
+include(${ZEPHYR_BASE}/samples/net/common/common.cmake)

examples/zephyr/test-cpp-src/Kconfig

@@ -0,0 +1,8 @@
+# Private config options for coap-client sample app
+
+# Copyright (c) 2020 Intel Corporation
+# SPDX-License-Identifier: Apache-2.0
+
+mainmenu "Test cpp build application"
+
+source "Kconfig.zephyr"

examples/zephyr/test-cpp-src/prj.conf

@@ -0,0 +1,4 @@
+
+CONFIG_LOG=y
+
+CONFIG_LIBCOAP=y

examples/zephyr/test-cpp-src/src/test.cpp

@@ -0,0 +1,21 @@
+#include <zephyr/logging/log.h>
+LOG_MODULE_REGISTER(coap_cpp_test, LOG_LEVEL_DBG);
+
+#include <zephyr/kernel.h>
+#include <errno.h>
+#include "coap3/coap.h"
+#include <stdio.h>
+
+coap_str_const_t *xxx_libcoap_get_version() {
+  coap_str_const_t *ver;
+
+  ver = coap_make_str_const(LIBCOAP_PACKAGE_VERSION);
+  printf("Version: %s\n", ver->s);
+  return ver;
+}
+
+int main()
+{
+  xxx_libcoap_get_version();
+  exit(0);
+}