Passes FC and F77 env vars when configuring Dune

Closes #146.

CMAKE_Fortran_.*  and environment variables related to Fortran are passed on
explicitly to Dune via the cache script DuneVariables.cmake. Since Fortran is
not a BEM++ languages (no enable_language(Fortran) in BEM), the variables
CMAKE_Fortran_.* will exist only if the user declares them explicitly.
Merge pull request #147 from bempp/rsdt/issues/146

Passes FC and F77 env vars when configuring Dune