Well, Borland had the benefit of being in control of both compilers :-). C++ Builder even had Object Pascal compatible extensions to link against VCL (which was written in Pascal).

You can already mix Free Pascal with C/C++ code if you use the same linker and libraries - i did manage to get FLTK statically linked with an initial backend for Lazarus in fact - this shot[0] shows a form and a few buttons from a self-contained binary on Linux (it links against FLTK statically and X11/etc dynamically).

But it is a PITA to get working for C++ libraries specifically (and you need a C intermediate for FPC to use). Also i couldn't get it to work for Win32, only Linux.

[0] http://runtimeterror.com/pages/iv/images/8a6ff400ed9b0d424be...