Setting HYPRE BoomerAMG parameters

Question

I am running a CPP version of the poisson equation and want to modify some of the HYPRE parameters. In petsc4py I could do something like this:

python myprogram.py -pc_hypre_boomeramg_strong_threshold 0.75

but when I try to do something like this in the CPP version of FEniCS:

./myprogram --petsc.pc_hypre_boomeramg_strong_threshold 0.75

It doesn't work. Here's my code:

#include <dolfin.h>
#include "CG1.h"
#include "papi.h"
#include <math.h>

using namespace dolfin;

class Source : public Expression
{
  void eval(Array<double>& values, const Array<double>& x) const
  {
    values[0] = 12*M_PI*M_PI*sin(2*M_PI*x[0])*sin(2*M_PI*x[1])*sin(2*M_PI*x[2]);
  }
};


class DirichletBoundary : public SubDomain
{
  bool inside(const Array<double>& x, bool on_boundary) const
  {
    return x[0] < DOLFIN_EPS or x[0] > 1.0 - DOLFIN_EPS or x[1] < DOLFIN_EPS or x[1] > 1.0 - DOLFIN_EPS or x[2] < DOLFIN_EPS or x[2] > 1.0 - DOLFIN_EPS;
  }
};

int main(int argc, char* argv[])
{
  // Initialize stuff
  int seed=80;
  //seed = atoi(argv[1]);

  // Create mesh and function space
  auto mesh = std::make_shared<UnitCubeMesh>(seed,seed,seed);
  auto V = std::make_shared<CG1::FunctionSpace>(mesh);  

  // Define boundary condition
  auto u0 = std::make_shared<Constant>(0.0);
  auto boundary = std::make_shared<DirichletBoundary>();
  auto bc = std::make_shared<DirichletBC>(V, u0, boundary);

  // Define variational forms
  CG1::BilinearForm a(V, V);
  CG1::LinearForm L(V);
  auto f = std::make_shared<Source>();
  L.f = f;

  // Define linear algebra
  auto A = std::make_shared<Matrix>();
  Vector b;
  KrylovSolver solver("cg", "hypre_amg");
  //KrylovSolver solver(argv[2],argv[3]);
  parameters.parse(argc,argv);
  solver.parameters["relative_tolerance"] = 1.0e-7;
  PETScOptions::set("ksp_converged_reason");
  PETScOptions::set("ksp_view");
  //PETScOptions::set("pc_hypre_boomeramg_strong_threshold", 0.75);
  Function u(V);

  // Compute solution
  assemble_system(*A, b, a, L, {bc});
  solver.set_operators(A,A);
  solver.solve(*u.vector(),b);

  // Save solution in VTK format
  File file("poisson.pvd");
  file << u;

  // Plot solution
  //plot(u);
  //interactive();

  return 0;
}

Any ideas what to do? I am using DOLFIN version 2016.2.0, here's my UFL file if needed:

element = FiniteElement("Lagrange", tetrahedron, 1)
u = TrialFunction(element)
v = TestFunction(element)
f = Coefficient(element)
a = inner(grad(u), grad(v))*dx
L = f*v*dx