I found I needed to predefine the VER variables at the top as ints, which I did by assigning them all to 0, to get this to work with the 6.1.2 compiler.
as in:
#define IDP_VER_MAJOR 0
#define IDP_VER_MINOR 0
#define IDP_VER_REV 0
#define IDP_VER_BUILD 0
I also had to update the very bottom with this:
#ifdef UNICODE
#include ".\unicode\idplang\default.iss"
#else
#include ".\ansi\idplang\default.iss"
#endif
but that might be because I include the idp.iss directly, not via an include folder.