I tried to install FreeFem++ 4.1 in Mint 19.1 (Ubuntu 18.04 derivative) and the make process stopped with
Making all in mpi
make: Entering directory ‘/usr/local/src/FreeFem-sources-4.1/plugin/mpi’
make: *** No rule to make target ‘…/seq/ff-c++’, needed by ‘MUMPS.so’. Stop.
So, ff-c++ was needed before it was created. I modified the first line in plugin/Makefile.am into
SUBDIRS=ffddm seq mpi hpddm
and now make works fine.