The preprocessor requires Windows users to pass the name of the compiler on their system when using use_dll. This is not necessary on Octave, as evidenced by matlab/utilities/general/dyn_mex.m. Fix preprocessor/ModFile.cc appropriately.

See discussion here: https://github.com/DynareTeam/dynare/commit/accd70a4c79c3e8dffb8ab367e1879680bd20606#commitcomment-19427662