g++ 5.3 expected initializer before <function> - c++

Question

I understand that this error is typically a syntax problem. I've gone through this with a fine toothcomb and can't spot it. The example below is whittled down to a couple of files which display the problem. The header files describe an API that is provided as a couple of lib files which I've referenced in the Eclipse project, but just trying to compile the following from the command line gives the same error.
I have several chained includes before getting to the first function declaration which throws an "expected initializer" error, along with every other declaration that follows.
main.cpp
#include <iostream>
#include <string>
#include <windows.h>
#include "defn.h"
using namespace std;
int main()
{
cout << "hello world" << endl;
return 0;
}
defn.h
#ifndef GEO_DEFINITIONS_H_INCLUDED
#define GEO_DEFINITIONS_H_INCLUDED
#include <iostream>
#include <string>
#define C_MICROSOFT
#define _UNICODE
#include "gxlib.h"
#endif // GEO_DEFINITIONS_H_INCLUDED
gxlib.h
#pragma once
#include <windows.h>
#ifdef C_MICROSOFT
#define GX_WRAPPER_FUNC __declspec(dllexport)
#define GX_STANDARD_FUNC
#define GX_WRAPPER_CALL _cdecl
#define GX_STANDARD_CALL _stdcall
#define GX_OBJECT_PTR void*
#define GX_VAR
#define GX_CONST const
#define GX_VOID void
#define GX_LONG long
#define GX_DOUBLE double
#define GX_HANDLE long
#define GX_LONG_PTR long*
#define GX_DOUBLE_PTR double*
#define GX_HANDLE_PTR long*
#define GX_ASTR_PTR char*
#define GX_WSTR_PTR wchar_t*
#if defined(GEO_UTF8)
#define GX_STR_PTR GX_ASTR_PTR
#elif defined( _UNICODE)
#define GX_STR_PTR GX_WSTR_PTR
#else
#define GX_STR_PTR GX_ASTR_PTR
#endif
#endif
#ifdef __cplusplus
extern "C" {
#endif
/*---------------- Copy_3DN[_public] ----------------*/
GX_WRAPPER_FUNC GX_LONG GX_WRAPPER_CALL
Copy_3DN(GX_VAR GX_OBJECT_PTR,
GX_CONST GX_HANDLE_PTR,
GX_CONST GX_HANDLE_PTR);
#ifdef __cplusplus
}
#endif
The command line and exact error is
c:\Code\CPP>g++ -std=c++0x -O0 -g3 -Wall -c -fmessage-length=0 -o main.o main.cpp
In file included from defn.h:11:0,
from main.cpp:4:
gxlib.h:55:1: error: expected initializer before 'Copy_3DN'
Copy_3DN(GX_VAR GX_OBJECT_PTR,
As I mentioned above, this is just an excerpt from a larger project that I'm building in Eclipse Mars using the MinGW-64 5.3 toolchain. Within Eclipse I've defined all of the include files and linked to the libraries, but this simple commandline example with everything in the same directory demonstrates the error.

Answer 1

I was lead to the answer by #SamVarshavchik's comment and this thread. The problem is the MSVC specific syntax
#define GX_WRAPPER_FUNC __declspec(dllexport)
#define GX_STANDARD_FUNC
#define GX_WRAPPER_CALL _cdecl
#define GX_STANDARD_CALL _stdcall
In my case the MSVC definitions are wrapped within the C_MICROSOFT #ifdef block, and that is the ONLY occurence of that definition, so the simplest solution is to remove the C_MICROSOFT define in defn.h and replace it with:
#ifdef __GNUC__
#define GX_WRAPPER_FUNC __attribute__ ((dllexport))
#define GX_STANDARD_FUNC
#define GX_WRAPPER_CALL __attribute__((cdecl))
#define GX_STANDARD_CALL __attribute__((stdcall))
#define _stdcall __attribute__((stdcall))
#define _cdecl __attribute__((cdecl))
#define GX_OBJECT_PTR void*
#define GX_VAR
#define GX_CONST const
#define GX_VOID void
#define GX_LONG long
#define GX_DOUBLE double
#define GX_HANDLE long
#define GX_LONG_PTR long*
#define GX_DOUBLE_PTR double*
#define GX_HANDLE_PTR long*
#define GX_ASTR_PTR char*
#define GX_WSTR_PTR wchar_t*
#define GX_STR_PTR GX_WSTR_PTR
#else
#define C_MICROSOFT
#endif