Add a test with a cpp executable including all mbed TLS headers

In case of any problems with the 'extern "C"' directives,
building the executable will fail
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0ade1d4..2a10d7c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,5 +1,5 @@
 cmake_minimum_required(VERSION 2.6)
-project("mbed TLS" C)
+project("mbed TLS" C CXX)
 
 option(USE_PKCS11_HELPER_LIBRARY "Build mbed TLS with the pkcs11-helper library." OFF)
 option(ENABLE_ZLIB_SUPPORT "Build mbed TLS with zlib library." OFF)