calling C++ functions from PostgreSQL

The PostgreSQL Programmer's Guide, Section 9.5, says "User-defined
functions can be written in C (or a language that can be made compatible
with C, such as C++)". Can someone point me to some examples or docs on
how to make C++ compatible with C? Despite searching, I'm unable to find
anything comprehensive that discusses calling C++ functions from C programs.

Specifically, our C language pg functions depend heavily on a second C
library (also used by other C applications) for functionality that isn't
specific to pg (such as complex string manipulation). I'd like to
migrate that generic C library to C++, but that requires calling the C++
routines, directly or through intermediate C functions, from pg.