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.

include/oscore/oscore.h

@@ -51,6 +51,10 @@
 #include "oscore_cose.h"
 #include "oscore_context.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /**
  * @ingroup internal_api
  * @addtogroup oscore_internal
@@ -124,4 +128,8 @@ void oscore_roll_back_seq(oscore_recipient_ctx_t *ctx);
 
 /** @} */
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* _OSCORE_H */

include/oscore/oscore_cbor.h

@@ -49,6 +49,10 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /**
  * @ingroup internal_api
  * @defgroup oscore_cbor_internal OSCORE CBOR Support
@@ -141,4 +145,8 @@ uint8_t oscore_cbor_strip_value(const uint8_t **data, size_t *buf_size,
 
 /** @} */
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* _OSCORE_CBOR_H */

include/oscore/oscore_context.h

@@ -52,6 +52,10 @@
 #include "coap3/coap_uthash_internal.h"
 #include <stdint.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /**
  * @ingroup internal_api
  * @addtogroup oscore_internal
@@ -278,4 +282,8 @@ int oscore_derive_keystream(oscore_ctx_t *osc_ctx,
 
 /** @} */
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* _OSCORE_CONTEXT_H */

include/oscore/oscore_cose.h

@@ -48,6 +48,10 @@
 
 #include <stdint.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /**
  * @ingroup internal_api
  * @defgroup oscore_cose_internal OSCORE COSE Support
@@ -267,4 +271,8 @@ int cose_encrypt0_decrypt(cose_encrypt0_t *ptr,
 
 /** @} */
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* _OSCORE_COSE_H */

include/oscore/oscore_crypto.h

@@ -50,6 +50,10 @@
 
 #include <coap3/coap_internal.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /**
  * @ingroup internal_api
  * @addtogroup oscore_internal
@@ -140,4 +144,8 @@ int oscore_hkdf(cose_hkdf_alg_t hkdf_alg,
 
 /** @} */
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* _OSCORE_CRYPTO_H */