omp parallel for in Conv3D and ConvGrad3D

Actually I don't understand what's going on with the build failure and c_compile_args.
The superclass method should be called automatically by python semantics.
The failures in that file have nothing to do with the changes I made, e.g.
ERROR: test_alloc_diag (theano.sandbox.linalg.tests.test_linalg.test_diag)