HAVE_MPI in ssg.h
In a code using SSG (branch ssg-interface-rewrite), one has to #define HAVE_MPI before calling #include <ssg.h> to enable MPI, but that does not guaranty that SSG was actually compiled with MPI.
Solution: there should be an ssg-config.h generated from a ssg-config.h.in and installed along ssg.h. This file would #define SSG_HAVE_MPI (or not) depending on how SSG was configure. ssg.h should then #include <ssg-config.h> and use #ifdef SSG_HAVE_MPI instead of HAVE_MPI.