C++: Declaration of variable in global scope conflicts with declaration with C language linkage

I have some legacy code of InDesign Plugin that I am trying to compile for new version of InDesign that uses C++11 supported compiler with XCode.

There is a header file AdWrksSession.h that contains a class object with C linkage as following. As declaration of an extern object/variable goes:

extern "C" CAdWrksSession gAdWrksSession;

and the cpp file AdWrksSession.cpp contains a global class object with Default constructor of the same name as the extern object :

CAdWrksSession gAdWrksSession;

The older compilers were not showing any error message for this but when using it with new compiler provided by Apple for XCode 7.2.1 with C++11 support it shows me the following error:

Declaration of 'gAdWrksSession' in global scope conflicts with declaration with C language linkage

I have tried following already asked questions for C linkage:

Compiler error for conflicting variable declarations: "conflicts with new declaration with 'C' linkage"

What is "extern linkage and with C language linkage"

Mixing declarations with extern, static and no storage specifier in global scope

I tried following syntaxes for extern variable declaration in header file:

#ifdef __cplusplus
   extern "C" {
#endif
        CAdWrksSession gAdWrksSession;
#ifdef __cplusplus
    }
#endif

and

#ifdef COMPILE_FOR_C_LINKAGE
     extern "C" CAdWrksSession gAdWrksSession; 
#else
     extern "C++" CAdWrksSession gAdWrksSession;
#endif

but nothing has worked so far.

When I make the header file declaration as static the code builds successfully but the feature doesn't work, of course.

Can anyone help me here. Thanks.