Add option smpi/privatize-lib to add extra library to privatization.

Add option smpi/privatize-lib to add extra library to privatization.
This can be use for instance with libgfortran for fortran codes with I/O.
TODO :
- pass multiple libs (miniDFT may need this)
- find a better way to find the lib, for now we have to give the exact lib name or path
  (libgfortran.so.3, or /path/to/libgfortran.so.3 works, but not libgfortran or libgfortran.so)