In one header file (shared.h):
extern int this_is_global;
In every file that you want to use this global symbol, include header containing the extern declaration:
#include "shared.h"
To avoid multiple linker definitions, just one declaration of your global symbol must be present across your compilation units (e.g: shared.cpp) :
/* shared.cpp */
#include "shared.h"
int this_is_global;