added functions to generator for setting compile flags and for unconditionally making targets

Can be used directly in a scipt by calling for example
  dune.generator.setFlags("-g") or dune.generator.unsetFlags() to get the flag used during configure
and
  dune.generator.setNoDependencyCheck() and dune.generator.setDependencyCheck()
before/after a module is compile, e.g., calling dune.grid.yaspGrid(...)