Wrong syntax for MPICH

While installing/compiling LIGGGHTS with "make auto", if the "mpicxx" command from MPICH distribution is set to use other compiler than g++ (say icpc for instance), the makefile will try to change back to g++ using the syntax:
"mpicxx file.cpp -cxx g++"

However this syntax is incorrect (at least in the latest MPICH version, 3.2.1) and causes error. The correct is:
"mpicxx file.cpp -cxx=g++"