Use externalproject_add to manage mod->hpp depenedencies.

Builds modcc from modparser as an external project;
executable will reside under BUILD/external/bin where
BUILD is the CMake build directory.

CMake hackery (see 'build_all_mods' target) should generate
required module .hpp files from .mod files before the
cellalgo library is built, if the header files are
missing or out of date.

This should also suffice for performing out-of-source
builds.