MXS-1842 Compile all authenticators as C++

Minimal changes, only what is needed to compile.

server/modules/authenticator/NullAuthAllow/CMakeLists.txt

@@ -1,4 +1,4 @@
-add_library(nullauthallow SHARED null_auth_allow.c)
+add_library(nullauthallow SHARED null_auth_allow.cc)
 target_link_libraries(nullauthallow maxscale-common mysqlcommon)
 set_target_properties(nullauthallow PROPERTIES VERSION "1.0.0")
 install_module(nullauthallow core)

server/modules/authenticator/NullAuthAllow/null_auth_allow.cc

@@ -43,6 +43,8 @@ static bool null_auth_is_client_ssl_capable(DCB *dcb);
 static int null_auth_authenticate(DCB *dcb);
 static void null_auth_free_client_data(DCB *dcb);
 
+extern "C"
+{
 /**
  * The module entry point routine. It is this routine that
  * must populate the structure that is referred to as the
@@ -89,6 +91,7 @@ MXS_MODULE* MXS_CREATE_MODULE()
     return &info;
 }
 /*lint +e14 */
+}
 
 /**
  * @brief Null authentication of a user.